The function continues the series: StrLeftBack, StrRightBack.
It should work like InStr, but starting from the end and moving to the beginning.
This function is really not enough when working with large amounts of data.
N.B.: Must also be considered: https://domino-ideas.hcltechsw.com/ideas/IDEAAD-I-139
In his book "The [unfinished] LotusScript Book", Julian Robichaux showed examples for faster algorithms.
In any case, the code for this function has already been written and is used in StrLeftBack, StrRightBack. To make it a separate function is quick and easy.