The BU0836A interface board accommodates up to 8 axes and 36 buttons, four of which are grouped into a hat button arrangement. If you don't need a hat button, this is a waste of four ordinary buttons. FSX, P3D and LINDA don't offer a solution for this. FSUIPC, however, provides a possibility to assign some behaviour to any of the four buttons, as if it were ordinary buttons.
LINDA got me programming my button behaviour in LUA. The assignment of LUA functions to buttons in LINDA is much more elegant and better maintainable then calling LUA files in FSUIPC.
There are at least two ways to get around this limitation in LINDA: buy another BU0836A (not expensive anyway) or store your LUA procedure without enveloping function ... end block as a conveniently named file with the extension ".lua" in the Modules folder of FSX or P3D and assign this behaviour with "LUA " to some button in FSUIPC. I got my "ordinary" buttons 32 to 35 working in this way.
Yet, for reasons of uniformity and maintainability, I would prefer a solution comparable to the one of FSUIPC in LINDA.