Why frigates usually stayed out of the line of battle

When did frigates gain the convention that ships of the line should not fire on them during a fleet action?

The convention applied mainly to fifth rates and smaller ships from around 1700; fourth rates remained powerful and similar enough to ships of the line that they could still be engaged.

Smaller fifth- and sixth-rate frigates increasingly stayed outside the main line and were not treated as primary opponents. Fourth rates occupied a dangerous boundary: they looked and fought enough like ships of the line that they might have to hang back or accept battle. The distinction reinforces how broad the historical category frigate could be.