The following is a combobox with values in the list of item 1 item 2 item 3 and do on. Upon selecting Item 1 the following code will fire in onchange, however selecting different items AFTER item 1 will cancel that out and set the value to item 1 not selected.
If(ComboBoxIncidentCategories.Selected.Value1="Item 1", Set(fireVar,"Item 1 Selected"),Set(fireVar,"Other Item Selected"))
the following change to above code will run it every time a change is made as long as item 1 is selected, regardless if something is chosen after it.
If(CountRows(Filter(ComboBoxIncidentCategories.SelectedItems,Value1="Item 1"))=1, Set(fireVar,"Item 1 Selected"),Set(fireVar,"Other Item Selected"))
Both have their uses, so adapt the one you prefer