The Web Elements Section allows you to control how the Drop Down lists appear for the Contact Record, Contact Record – Apply Actions, Contact Search and Action Sets (Add/Remove Tag) pages.  Basically, the option replaces the standard drop down list into a searchable version.

Example picture below shows a searchable single selection drop down list.

KB - Infusionsoft - Global - Web Elements Drop Down

 

Example picture below shows a searchable multiple selection drop down list, with 2 items already selected.

KB - Infusionsoft - Global - Web Elements Drop Down Multiple