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.
Are you referencing a book that is teaching braille (the actual braille sign for ch) or are you referencing a book that is teaching grammar? In a grammar book, I would use the uncontracted letters ch as the heading. No grade 1 indicator required and no dot locator required. If this is a braille lesson and the braille sign for ch is being taught, I would do the ch uncontracted and then follow it with the contraction to demonstrate the concept.
This is a book that is teaching braille -- the student is learning the actual braille sign for ch (dots 16). The heading for the page is simply (dots 16) which of course will read as "child" in this contracted environment. Another page heading is "his was were" which can be misread as "have just go" or perhaps as punctuation (although the Grade 1 indicator would be needed there, and 2356 now has no Grade 1 meaning) ... so I thought dot locators would be needed there as well. I think I'll go with dot locators for "use". Thanks for the conversation.