[terran_emperor]

What does special Hitpoints do?

Does this feature allow you to customise the hull strength of a ship making is stronger or weaker without the need for TBL hacking?

[Shade]

Precisely that.

[FUBAR-BDHR]

Also shields if the ship has them.  One thing to watch out for if you start using a lot of them is they use up 2 variable slots for each one.  Also removing them doesn't always remove them.  Kind of keep an eye out on the variable section using notepad or whatever to be on the safe side.  Set up right they are very useful though. 

[Stormkeeper]

2 variable slots ? What do you mean ? Fred uses variable slots to track ship modifications ? Then how many variable slots are there?

[FUBAR-BDHR]

FRED uses variables for a few things.  The most obvious is variables you create in the events editor.  The other 2 I know of are special hits and I think special explosions.  There are 100 slots.  Regular variables count up from 0 - 99.  Block variables (the kind used by special hits) count backwards. 

It's not very often that they all get used.  Problems arise since there is no way of deleting them in FRED.  So if yo do a lot of adding and deleting ships while messing with special hits you can use them up real quick.