A Combo Box in InfoPath is a cross between a Text Box and a Drop-Down List Box. It lets the user choose one of the drop-down choices or enter free text. The user can also pick one of the choices and then modify it.
Formotus doesn't support the Combo Box control. But don't worry, you can use this trick instead to create an even better mobile interface. Download this sample form to see how you can use a Dropdown List Box to populate a Text Box with one or more items that are then editable.
In this example a mobile worker is reporting on a job he was dispatched to. Rather than typing in text he can choose from a drop-down list:
With three taps on the Drop-Down List he can add three different text chunks into the Text Box:
This technique has great potential for enabling mobile workers to enter detailed descriptions without much keyboarding. It's easy to create with a concat() rule that appends the new text to the existing text, like this: