ComboBox control allows the user to choose from a list of valid choices. The ComboBox displays the selected choice. When tapped in New/Edit mode, the ComboBox displays a popup which allows the user to make a choice from a list.

The ComboBox control shows values which are either  from a value list or retrieved from another table.

a) “Fixed Valued List Combo Box” : Source Type:"Valued List"
Use this If you want to choose value from set of fixed predefined value.
Combobox field type will pop up a list of fixed predefined choices when selected,  Also You can make sure the value can only be one of the popup choices by setting "Allow Item Edit" to "False"

b) “Lookup Combo Box” : Source Type:"Another Profile"
A lookup column (or field) is a field in a table whose value is retrieved from another table . You can use a lookup column to display a list of choices in a combo box. The choices can come from a table. You can create a lookup combo box by setting a control's "Source Type" property to "Another Profile", "Lookup Profile" to profile from which you want to get values and "LookUp Column" to column from lookup profile whose content should be used as a list.

      Two modes of “Lookup Combo Box” are as follows.

1. “Simple Lookup Combo Box” : For using values from lookup column in another profile. ( “Lookup Condition” property set to Blank)
2. “Conditional Lookup Combo Box” : For using refined values (Filtered values) from lookup column in another profile. ( “Lookup Condition” property set to desired filter criteria)

Please find the attached document for more information.