.xla, Then Tools | Addins..., to add the file.

.xla is the format for Excel add-in.

However, it does not necessarily hide your code in VBA, if that was what you
want.

Whether it is .xls or .xla, you can only "hide" VBA code by
password-protecting the VBA project, which is a weak protection.

If you really want to hide your VBA code from viewing by others, the better
approach would be to move most of your code logic into a ActiveX DLL, and
use this ActiveX DLL in your VBA project. In this case, your VBA code would
be simplified to just a few simple macros that calls object/methods in the
dll, which code, of course, is not viewable.

When you build the ActiveX dll with VB(5/6), you can simply code your VBA
code into VB DLL project, only minor revision is required, depending on what
the code does.