Biaxial fiberglass cloth tape is available with or without mat (chopped strand mat, or CSM) backing. The mat backing is a non-woven material made of randomly placed, chopped glass fibers held together by a binder.
When wetting out biaxial fiberglass tape, the epoxy will saturate the cloth tape and make it transparent.
However, when wetting out biaxial fiberglass tape with mat backing, the epoxy will saturate the tape, but the tape will not become completely transparent because epoxy does not completely break down the binder holding the mat together. Epoxy will not form the best bond to the mat backing, but overall strength should not be an issue, as the tape will be suspended in the cured epoxy.
To wet out biaxial fiberglass tape with mat so it is completely transparent, you have to use polyester resin. The styrene in polyester resin breaks down the binder in the mat.