If you check the Warn me when the number of characters in the translation is... checkbox, the number of characters will be checked according to the settings made below. You can set and activate a lower tolerance limit and an upper tolerance limit. The default values are 100% for the lower limit and 100% for the upper limit.
The default values may seem to make no sense as they do not leave any room for a difference. However, this function does make sense under very special circumstances, namely when there is the formal requirement that the number of characters in the translation must not be higher/lower then the number of characters in the source text. This can be the case in software localization projects.