I’m super confused when to consider a positive root on GRE vs. when to consider both negative and positive roots.

Some books mention that when ETS explicitly gives a (root) symbol, we only take positive root. If, however, you encounter square root whilst you solve a problem, you consider both.

But in a couple of Gregmat questions, I had to only consider positive root for the latter condition as well.

Can someone clarify this for me with an example. Much appreciated!