At a guess, I'd say that a senior stiff shaft would probably have similar characteristcis to a regular (in terms of weight and flex) but a mid kick point. Most regular shafts are likely to have a low kick point (to get the ball in the air). Only a guess...

As for stiff "lite"....any shaft can be designed to be light/lite regardless of flex. Stiff steel shafts are heavier than regular in general, so a bit like the answer above, I guess stiff lite would weigh a similar amount to a regular but with stiff characteristics of kick point and torque....also, only a guess.

My graphite design YS stiff shafts (in my irons) are far lighter than the regular steel of my old/spare set.
They have a mid kick point to keep a lower trajectory (presumably) and a larger tip (think "fat shaft") to improve the twisting of a narrow tip.

So what with flex, weight, kick point, torque etc. there are getting on for 24 variations without even starting. Hence the reason for trying before you buy.