Recent studies show that it is better to select the trophectoderm. If you have 2 blastocysts and 1 is with a good ICM, and the other doesn’t have a good ICM but has a very good trophectoderm, you choose the one with a very good trophectoderm.
It is up to the embryologist to choose the embryo in this situation, depending on how it is done in this specific lab. It is very important because the ICM will develop the embryo afterwards, but you have to have something that is equilibrated between the ICM and the trophectoderm. Sometimes it’s difficult to decide, you have to see the blastocysts to decide, but the latest studies show the best implantation rate is related to the best trophectoderm.