If I remember well, SplayAz takes the input array and set the position of each signal through this expression:

var pos = [ center - spread, center + spread ].resamp1(n);

so, if n = Array.size = 4 ---> pos = [center-spread, center-spread/3, center+ spread/3, center+spread].
if n = Array.size = 3 ---> pos = [center-spread, center, center+spread](because pos[1] is the mean between) and spread does not affect pos[1].

Given the instant position of any signal, it make the sum of each output.

So, with an Array of size 2, with spread=0, the signals move together:

(
x = { arg spread=1, level=1, width=2, center=0.0;
 b = Array.new(3);
 b.add(SinOsc.ar(300, 0, 1));
 b.add(Saw.ar(300, 1));
 SplayAz.ar(
  8,
  b,
  spread = 0,
  level,
  width,
  center = SinOsc.kr(0.1), levelComp: false
 );
}.scope;
)

and, with a varying spread, the signals move in opposite directions, one spreading over the left side and the other over the right one.

(
x = { arg spread=0, level=1, width=2, center=0.0;
 b = Array.new(3);
 b.add(SinOsc.ar(300, 0, 1));
 b.add(Saw.ar(300, 1));
 SplayAz.ar(
  8,
  b,
  spread = SinOsc.kr(0.1),
  level,
  width,
  center = 0, levelComp: false
 );
}.scope;
)

Here, of course, we have a Sinusoidal control, therefore there is this accordion effect.
Any way, when pos = 1 or pos = -1, the signal Array[i] is sent to (output.size/2 + i).

Ok, now if, as you say:

so, if n = Array.size = 4 ---> pos = [center-spread, center-spread/3, center+ spread/3, center+spread].
if n = Array.size = 3 ---> pos = [center-spread, center, center+spread](because pos[1] is the mean between) and spread does not affect pos[1].

Each pos is then interpreted as a position in PanAz (which are summed up for each channel).

^PanAz.ar(numChans, inArray.asArray, pos, level, width, orientation).flop.collect(Mix(_))

OK, so spread relates to the position range of the PanAz [-1...1]

Improved documentation.