It's probably only optional variant parameters that Excel makes visible.

More specifically, if you step into a sub with an option variant parameter
from the macro popup or directly in the VBE, the parameter has a type of
Variant/Error and is listed as missing..

But if you go into the VBE and try to step into a sub with an optional
string or integer parameter, you'll find that Excel won't do anything.

Although the missing variant parameter supposedly has type "Error", a sub
with an optional parameter of type Error doesn't get in the list.

As I understand it, variant parameters are the only ones that will actually
be marked as missing if omitted. Optional parameters of other types will be
initialised to zero, empty, etc, when omitted. For some reason, this seems
to make a difference.