B2 multiple entry?

My mother-in-law got her B2 yesterday. The visa page showed an "M" under entry, and the visa is valid for 6 months. I wonder if this means she can use this visa to enter the US for several times, as long as within this 6 months.

It seems that B2 is a single-entry visa. Is it normal that her B2 has multiple entry?

Thank you so much.