An obligatory scene is one that has to take place. Like in a romance, there has to come a point where the lovers fall into each other's arms and realize they are meant for each other.
Pattison says, The obligatory scene is usually one of he most emotionally powerful. It often contains a revelation, a complication, a reversal, a twist, a new meaning for old events, or something else that surprises the reader in some way ... It is is often a cascade of emotional confrontations taking conflicts between different characters (each with their own subplot) in turn and resolving them."
This helped bring into focus something that was only vaguely nagging at me.