How to understand the term 'unit' in 'unit-at-a-time'?

[QiangHuang]

      Hi Everyone:

      GCC now has two styles for compilation, one for
'function-at-atime', and the other for 'unit-at-a-time'.
      The term 'unit' in this word refers to a whole source file ? or
something else.

      Could someone make an exact definition about it?
      And How the compiler make the decision that if the functions are
in the same compilation unit.

      Thanks a lot~