The way it works is like this:
There used to be 2 scales one was "A" ( the lower scale )and the other was "B" the higher scale. Now the "A" scale is the normal salary scale, and the "B" scale is known as the "with productivity differential" scale. The latter is only paid to the top proportion of the pilots on the company pilots seniority list. It is not likely to apply to anyone who has been there for much under 10 or 11 years. The WPD scale is 11.25% higher than the normal salary scale. When you eventually qualify for it you move across at the appropriate grade level ( probably 10 or 11 at this time).
On both scales there are yearly point grade increments. 14 for Captains and 13 for Senior First Officers. There are also 5 incremental grade points for First officers and a further 3 lower incremental grade points for Cadet entry pilots.
The cap works like this :
If you joined before the end of 2004 you made your way right up the relevant rank grade point increment until you were promoted then you either switched across to the basic Captains basic grade point increment or in certain circumstances at a higher grade point on that scale.
Since the end of 2004 you join on the First Officers basic scale and follow that upwards each year for 5 years ( unless you are promoted when you join the basic Captains scale ) and then you join the Senior First officers salary scale and follow that up to year 5 at which point it is capped ( pre 2004ers go up to year 13). When you are promoted you follow the Captains basic scale ( or WPD scale when it eventually applies to you ) but only up to the cap at year 10 (pre 2004ers go upto year 14). The cap for new joiners is therefore at the end of years 5 for Senior First officers ( which is in fact 10 points up the scale if you include the F/O and SFO scales together, and the end of year 10 for New joiner Captains.
The WPD scale at those same levels does apply to new joiners.
The only proviso that may be of interest in this regard is that after completing 6 years on the F/Os/ SFO's increment scale you are given a notional placing on the Captains scale. This means if you complete say 9 years as a First (senior First) Officer and are then promoted, you move across to year 2 on the Captains scale.
I hope this helps explain your question, because I have tried to simplify it. There are other provisos and conditions but these would not really be relevant to the basic question.