Reconcile with some of Paul's wording

studentdsx · Jan 22, 2018 · 684a518 · 684a518
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diff --git a/1.3/spec.md b/1.3/spec.md
@@ -11,19 +11,33 @@ MBTiles is a specification for storing tiled map data in
 MBTiles files, known as **tilesets**, MUST implement the specification below
 to ensure compatibility with devices.
 
+## Compatibility
+
+*This section is informative and does not add requirements to implementations.*
+
+Because views may be used to produce the MBTiles schema, two implementations
+may store tiles with different internal details, meaning one implementation
+may not be able to add to an existing file.
+
+As a container format, MBTiles can store any tiled data, so data can be stored
+that an implementation cannot do anything with.
+
+Relying on metadata keys not defined in the specification can cause
+compatibility problems.
+
 ## Database Specifications
 
-Tilesets MUST be valid SQLite databases of
+Tilesets SHALL be valid SQLite databases of
 [version 3.0.0](http://sqlite.org/formatchng.html) or higher.
-Only core SQLite features are permitted; tilesets MUST NOT require extensions.
+Only core SQLite features are permitted; tilesets SHALL NOT require extensions.
 
-MBTiles databases SHOULD use [the officially assigned magic number](http://www.sqlite.org/src/artifact?ci=trunk&filename=magic.txt)
+MBTiles databases MAY use [the officially assigned magic number](http://www.sqlite.org/src/artifact?ci=trunk&filename=magic.txt)
 to be easily identified as MBTiles.
 
 ## Database
 
 Note: the schemas outlined are meant to be followed as interfaces.
-SQLite views that produce compatible results are equally valid.
+SQLite views that produce compatible results MAY be used instead.
 For convenience, this specification refers to tables and virtual
 tables (views) as tables.
 
@@ -49,7 +63,7 @@ The metadata table is used as a key/value store for settings. It MUST contain th
 * `name` (string): The human-readable name of the tileset.
 * `format` (string): The file format of the tile data: `pbf`, `jpg`, `png`, or an [IETF media type](https://www.iana.org/assignments/media-types/media-types.xhtml) for other formats.
 
-`pbf` here refers to gzip-compressed vector tile data in
+`pbf` as a `format` refers to gzip-compressed vector tile data in
 [Mapbox Vector Tile](https://github.com/mapbox/vector-tile-spec/) format.
 
 The `metadata` table SHOULD contain these four rows:
@@ -104,9 +118,9 @@ The `zoom_level`, `tile_column`, and `tile_row` columns MUST encode the location
 of the tile, following the
 [Tile Map Service Specification](http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification),
 with the restriction that
-**the [global-mercator](http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification#global-mercator) (aka Spherical Mercator) profile is assumed**.
+the [global-mercator](http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification#global-mercator) (aka Spherical Mercator) profile MUST be used.
 
-Note that in the TMS tiling scheme, the Y axis is reversed from the coordinate system commonly used in the URLs
+Note that in the TMS tiling scheme, the Y axis is reversed from the "XYZ" coordinate system commonly used in the URLs
 to request individual tiles, so the tile commonly referred to as 11/327/791 is inserted as
 `zoom_level` 11, `tile_column` 327, and `tile_row` 1256, since 1256 is 2^11 - 1 - 791.
 
@@ -137,9 +151,9 @@ A typical create statement for the `grid_data` table:
 
 #### Content
 
-The `grids` table contains UTFGrid data, compressed in `gzip` format.
+The `grids` table, if present, MUST contain UTFGrid data, compressed in `gzip` format.
 
-The `grid_data` table contains grid key to value mappings, with values encoded
+The `grid_data` table, if present, MUST contain grid key to value mappings, with values encoded
 as JSON objects.
 
 ## Vector tileset metadata

diff --git a/README.md b/README.md
@@ -1,6 +1,6 @@
 # MBTiles Specification
 
-MBTiles is a specification for storing tiled map data in
+MBTiles is a specification for storing arbitrary tiled map data in
 [SQLite](http://sqlite.org/) databases for immediate usage and for efficient transfer.
 MBTiles files, known as **tilesets**, must implement the specification below
 to ensure compatibility with devices.
@@ -15,29 +15,29 @@ to ensure compatibility with devices.
 # Concept
 
 MBTiles is a compact, restrictive specification. It supports only
-tiled data, including image tiles and interactivity grid tiles. Only the
-Spherical Mercator projection is supported for presentation - tile display -
+tiled data, including vector or image tiles and interactivity grid tiles. Only the
+Spherical Mercator projection is supported for presentation (tile display),
 and only latitude-longitude coordinates are supported for metadata such
 as bounds and centers.
 
-It is a minimum specification - only specifying the ways in which data
+It is a minimum specification, only specifying the ways in which data
 must be retrievable. Thus MBTiles files can internally compress and optimize
 data, and construct views that adhere to the MBTiles specification.
 
 Unlike [Spatialite](http://www.gaia-gis.it/spatialite/), GeoJSON,
-and Rasterlite, MBTiles is not raw data storage - it is storage
-for presentational data, like rendered map tiles.
+and Rasterlite, MBTiles is not raw data storage. It is storage
+for tiled data, like rendered map tiles.
 
 One MBTiles file represents a single tileset, optionally including grids
-of interactivity data. Multiple tilesets - layers, or maps in other terms,
-can be represented by multiple MBTiles files.
+of interactivity data. Multiple tilesets (layers, or maps in other
+terms) can be represented by multiple MBTiles files.
 
 ## UTFGrid
 
 The MBTiles specification previously contained the
 [UTFGrid specification](https://github.com/mapbox/utfgrid-spec).
 It was removed in version 1.2 and moved into its own specification
-with synced version numbers - so MBTiles 1.2 is compatible with
+with synced version numbers, so MBTiles 1.2 is compatible with
 UTFGrid 1.2. The specs integrate but do not require each other
 for compliance.