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LDRA and LDRB just tell the hardware when to load the counter value into RA and RB. This is all about time measurement, which may or may not be used for PWM.
Page 868 shows RA, RB and RC used in PWM
This is the first time I've looked at the timer registers, so please forgive me if I've made a mistake 
They have uses in capture mode where your timing an external signal of some sort.
If you look at page 868, you will see that RC is the timer loop count (timer range) and RA and RB can be used to time 2 separate events on the same timer
senioriceman:
Hi Rufe0,Is it like measuring the high time of a input signal to the pin.
You can do that yes aswell as the time between pulses and other things. It's all explained in the manual
Page 856 onwards, it is abit to take in all at once but once you've read it a few times and used it in programs, you will understand it.