Okay. A modified thinset is one that has latex or acrylic polymers mixed into it to give it strength, both mechanically, as well as by slowing down the portland cement's curing process.

An unmodified thinset is simply one without polymers added. It can be mixed with water and used as an unmodified thinset, to bed cement board, or to set tile over Schluter's Ditra membrane. OR, it can be mixed with a liquid latex additive to modify and strengthen it.

Your awesome as always sir, thank you.