Declaring a lookup/combobox control
Choice Control (Single Selection)
Declare a new ComboBox or LookupPicker control. Set the Target Field Name and in the Lookup Details tab, set the Lookup Static Values property adding all the required values concatenated by semicolon (;).
For example for a rating field : Low;Medium;High
During initial form customization, the form designer identifies the choice controls and prepares the values for you.
Choice Control (Multiple Selections)
Declare a new MultiLookupPicker control, set the Target Field Name and set the Lookup Static Values property (values concatenated by semicolon). The MultiSelect propertyin the General tab should be checked.
For example : SALES;SUPPORT;SERVICE;MARKETING
Lookup Control (Single Selection)
Declare a new ComboBox or LookupPicker control
In the Lookup Details tab, set the Lookup Url (for example http://servername). As soon as the property is updated, the Lookup List combobox is filled with the available lists found. Select the required List and the required view (optional).
If you need you may set criteria to query the list in the field Lookup Criteria. The criteria are defined using the Criteria Editor.
Available values for the Operator are : Equals, NotEquals, Greater, GreaterOrEqual, Less, LessOrEqual, IsNull, IsNotNull, BeginsWith, Contains.
Available values for the type are : Text, Counter, DateTime, Lookup, Number
Next, select the Lookup Display Field (usually Title) and the Lookup Value Field (usually ID, unless you have selected to store a different field in your form).
More information on how to setup your lookup criteria can be found HERE.
If you decide to create a ComboBox field, all values are loaded during startup. For long lists, you should select the LookupPicker control. The LookupPicker allows the user to search for the required value by opening a popup dialog. If you still decide to present all the values during popup loading, you should check the Preload Lookup Data checkbox.