The first number is the ramp angle that the pawl acts against under drive/acceleration. The larger the number the steeper the angle which slows the locking action. The second number is the angle/slope the pawl faces under deceleration/braking. In most situations this is 90 degrees or a right angle, so the differential does not lock up under braking. There are some motorsport disciplines where locking under braking is desirable, such as loose surface rallying or drifting, where a reverse flick and destabilising the rear is used to aid turn in. In this situation a 40/40 ramp angle might be more suitable.
50/90 Ramp angle 25-35lb preset. This is a fast road set up that is more progressive and gentle in operation.
40/90 Ramp angle 35-45lb preset. Fast road, hillclimb use. This is the most common set up for roadgoing Sprites using 5” or 6” rims with treaded road legal tyres.
30/90 Ramp angle 65-75lb preset. Race use or competition use using slicks or very sticky list 1b 1c tyres. Avon ZZR, ACB10, AO48, R888 etc. Also used in some loose surface events where a faster more aggressive locking action is required.
A car used primarily for road/touring is probably better with a Torque biasing LSD as they are quiet in operation and are generally fit and forget. (Allowing for regular oil changes).
More competition focussed cars are generally better with a fully lockable LSD, they do require more maintenance as the clutch faces wear, and they do sound like a bag of spanners at slow speed with steering lock applied!.
The 3J LSD are adjustable, the different ramp angles are machined into a steel carrier within the unit, which can be unbolted and moved around to the position you require. The preset is also adjustable by turning,adding or removing the plates in the clutch pack. (3J offer this service if required).