This is a very hard concept for me to grasp...but I'll try!
The dot locator for mention is used whenever symbols are listed (on the Special Symbols page, in a TN, in a list that is in print).
The dot locator for use is used whenever a symbol (such as a free standing colon) might get "lost". The dot locator is to help the reader know which positions within the cells have dots in them (that it's a colon and not hyphen). Sometimes, a grade 1 indicator is used to designate punctuation standing alone (see the free standing question mark on page 51 of the UEB codebook). Also, you cannot have an indicator on a line by itself without using the Dot locator for use. See the examples on page 30 of the UEB codebook).
One of the dot locators is used only with symbols lists...the other is for when a symbol is actually being used for some purpose - not just being explained.
I hope that helps. If not, let me know and I will ask for more information from other sources.