DTTD is Titanium Dioxide which has been surface treated with Dimethicone. If you are working with silicone oil formulations, it is particularly easy to mix in, without a lot of fussiness. For example, we use this product in our TKB Matte Liquid Lipstick Base.
In general, a dimethicone treated powder has good water repelling qualities, a smooth and soft skin feel (not draggy), good adhesion, high compressibility and spreadability. 