Every terminal block has a "Custom Data" field that you can use to store data. This is how a lot of the inventory managers work.

It depends on what the custom block is doing.

"Custom Data" is used for player scripts (progtamable blocks) to use not for the blocks internal use for example the position of a piston

Would have been nice to state that example up front.

I write programmable block scripts. Every block type has a set of variables attached to it.

Script wise and to go with your example : Piston.CurrentPosition is its current position and is a floating point number. The actual offset of that information might match what is in the interface reference but I doubt it.