What are the usable values for "opportunity" descriptor in effects coding?

[Olodrin]

Is this just a free-form field or is there a set of allowable/functional values?

Thanks

[Trenloe]

You specify if an effect applies only to opportunity attacks - see the "Modifiers" that include "opportunity": https://www.fantasygrounds.com/wiki/index.php/5E_Effects#Modifiers Basically, only effects that directly apply to an attack (to hit) roll: ATK, AC, ADVATK, DISATK, GRANTADVATK, GRANTDISATK

If whoever is attacking clicks the "Opportunity" button in the modifiers window before rolling their attack, then any of the above listed effects which are specifically assigned as "opportunity" will kick in for the attack roll.