In the Natural Units environment, the global type function is replaced by a type function that converts any unevaluated arithmetic operators, equalities, or inequalities to their global equivalents.  The first argument is tested to check whether it is a valid unit name.

For more information about this function, see the global type help page.

type(a1 = 3, 'name=integer');

with(Units[Natural]):

:-type(a1 = 3, 'name=integer');  # this unexpectedly returns false

type(a1 = 3, 'name=integer');

:-type(m, name);

type(m, name);  # this unexpectedly returns 'false', because m is the meter