The 'Multiple Items Form' is just another term the Boys of Redmond added, for who knows what reason, with versions 2007/2010. Clicking on the MIF Icon simply automatically creates a Continuous View Form with the data arranged laterally, like a Datasheet View Form, based on the Record Source (Table or Query) that was highlighted when the MIF Icon was clicked.
Whether the type of Form can be updated depends, as do all types of Forms, on whether or not the Record Source can be updated.
Forms based on single Tables, or Forms based on Queries that are, in turn, based on single Tables, can be updated.
Forms based on Queries that are, in turn, based on Multiple Tables, sometimes can and sometimes cannot be updated. For reasons why Multi-table Queries cannot be updated, have a look at Allen Browne's article on the subject:
Microsoft Access tips: Why is my query read-only?
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Hope this helps!
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007