You're translating a large document into Spanish which includes dozens of repetitions of the following question:
How many are there?
There are two possible translations here, dictated by gender:
¿Cuántas hay? and ¿Cuántos hay?
Having a TM that doesn't allow multiple translations would mean either stopping each time the segment comes up and manually making the change, or fixing the resulting document after the translation has been completed, both tedious and unnecessary actions. If you use a Translate to Fuzzy command or pre-translate your document, a single translation would mean that the errors introduced need to be fixed later, increasing editing time.
A better solution is setting up your TM to allow multiple translations, so that one source segment has two (or more) translations. Then it's just a matter of choosing the one you want to use.
The steps to do this are detailed below.