Can someone explain embryo grading?

Just talked to the RE, one of my questions was about the unthawing process and what happens if the first one doesn’t unthaw properly. Do we have to wait another cycle which she said no, they’d then take another frozen embryo and unthaw.

She said our 4 frozen embryos are 4ab, 4bb (5 days) and two 5bb (6 days). What does this mean? What’s the difference? I know the first letter is inside and second outside, but what does the number mean?