From the manual: Light troops are allowed one free turn of up to 45 degrees and one free turn in any direction each turn. Beyond this, any turn costs 8 AP.
Based on the script, the first phrase means that in the +/- 42 (84 total - that is 45 and 90 degrees practically) degree sector a light unit is allowed a turn on any reachable tile. Beyond this sector any turn costs 8 AP.
Update 26.10.2020. Sorry, this applies to light units out of generals' command range (eg. in the General mod or in FoG2). For the original P&S - see my next post.
For different types of light units it looks like this in the open (the blue circle means a turn without spending APs, the yellow one - with spending 8 APs, on the other reachable tiles a turn is not allowed after movement during one turn)
Light horse

Dragoons

Light infantry




