If you have a slab, the tile can be adhered directly to the slab. However there is a product
" Schluter Ditra 1/8" Underlayment "
This can be applied to the slab with an adhesive and then the tile is cemented to the underlayment. If you need to pull up the tile floor, you can pull it up a lot easier with the underlayment in place.
I have done it before and was a lot easier than i thought it would be.
As far as the STC ..... that is in reference to sound and how it travels. Check with the manufacturer for specs. The building may have rules for tile floor installation. Usually an absorbant underlayment is used to eliminate noise travel like a cork board or rugs. Not sure how it works with tile. You may want to ask around to see if other residents have tile floors. Maybe they have some up to date info