Name what the reader must understand and what may remain open. Preserve uncertainty that serves the intended theme, but supply the causal links needed to interpret the character's choices. Test revisions against the stated aim rather than a universal rule about endings.
Worked example
An ending may leave forgiveness unresolved while clearly showing an apology being offered. Omitting whether the apology was ever sent creates a different uncertainty and may weaken the intended focus.
The Unsent Letter Draft
A writer wants the ending to leave open whether friendship will recover, while making the narrator's decision to apologize unmistakable. The draft ends: 'I folded the letter. The street was empty. By morning, something would be different.' Earlier scenes establish the conflict, but nothing shows whether the narrator sends or delivers the apology.