It is a fund that you put money into, and you can draw money out upon demonstrated need. A slush fund has a bit of a pejorative connotation to it. Given that this is a very well-run and well-managed operation, we wouldn't want to equate the two.
The way these special operating agencies are established, the Treasury Board has the right at different times to look into them. The passport office operates on the same principle, where users of a given program pay a fee and the money is put in a revolving fund to address the needs of those users.