Enabling functionality in elisp

[wael-zwaiter]

In elisp code one can enable a feature using either a numeric value (0, 1)
or with a boolean (t, nil).  What constructs are available if one wants to
use either method?

For instance, I can use

  (if signal

by defining

  (defcustom signal t

Some useful examples would help me, particularly for designing minor-modes.