There are various systems available to denote the fineness of a yarn. For the filament yarn like Polyster direct system is used which is mainly denier and Tex.polyester filament yarns suppliers

Denier: It is the number which tells the weight in grams of 9000 meter of yarn. It is denoted by alphabet D. So 210D Polyester means, that 9000 meter of polyester will weigh 210 grams.

Tex: Tex is also metric system of numbering. It is a number which tells us the weight in grams for 1000 meter of yarn. So 210T polyester will mean 1000 meter of polyester will weigh 210 grams.

having said this, the relation between Tex and Denier is as follows:

Denier (D) = Tex (T) x 9.

So a 210D polyester will be finer ( lesser liner mass/unit area) than 210T polyester.