I think that OnClick is probably one of the most confusing events, overall, in Access, simply because how it is triggered varies so much, and not always as common sense would dictate! The Form OnClick event pops when you click on the Record Selector at the side of a Record, not when merely clicking anywhere on the Form, as might be supposed.
Each Section of the Form, i.e. the Header, Detail and Footer Sections has its own OnClick event, triggered when clicking anywhere on the Section with the exception of Controls on the Section that have OnClick events of their own.
The OnClick event of a Combobox only pops when a Selection is made from the Control, not by merely clicking anywhere on the Control.
As I said, it varies greatly!
As to usage of the Form OnClick, I don't believe I've ever used it! When you click to move to another/new Record, the Form_Current event fires, and this is what I normally use when doing something when a Record first becomes active Record. Perhaps someone else has found a use for it the Form OnClick and will post it here.
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