Aside from being somewhat allergic, cat urine is some really evil stuff.

And/or market it as a great unit for people allergic to pet hair :-)

It can be supplemental but is not necessary.  

We are only in business to make money not provide affordable housing or supplement tenants rents.

I still have to supplement with electric and propane heat in the winter but they look better than window units from the outside.  

Some will not raise the rents of good tenants preferring to supplement their rent in the belief keeping them is preferred over charging full market rent with the risk they will move.

Stay under market if you have a undesirable property otherwise why would you supplement your tenants rent.

However, today I received a supplement list from my tenants, and it states all bedrooms have "Lots of marks, dents, and holes & scratches on all walls".

I can however say that the scenario you outline is not a wholesome one - agreeing to one rate and then paying some kind of supplement on the side.

It includes but is not limited to monies derived from Social Security benefits, supplemental security income, unemployment benefits, other retirement programs, child support, the Aged, Blind or Disabled Cash Assistance Program, Refugee Cash Assistance, and any federal, state, local government, private, or nonprofit-administered benefit program."