Forms allow not only to get (and use) data from record specified as Form Data Source, but from any other Data Source Record too. Here is an example:

1. Specify record to get data from. It can be performed in different ways:
   • Pass Record ID via form parameter, e.g.: {$FormParameters.myRecord}
   • Pass Record ID by updating some form variable with some action. It will be available as, e.g., {$Variables.myVariable}
   • Select Record from List or Related List, it will be available as, e.g., {$Form.myList}
   • Select Record from some Lookup. It will be available as, e.g., {$Form.myLookup}
     
2. Create a Global Action that will get necessary fields from specified record:
   • Create Global Action of Define Variable type
   • Set Variable type to Query
   • Select Object Name = your Data Source Object (yes, Data Source Object should be specified, so, if you need take data from different Data Source Objects, you should create several similar actions)
   • Select necessary Fields to get data from and specify action output variable names (1)
   • Set conditions to get only one necessary record (2): Record Id = %your action parameter variable%
     
3. Create a Form Action in your form:
   • Type = Global Action
   • Name = %your Global Action name%
   • Source can be empty
   • Variables:
     • Set Name = %your action parameter variable%
     • Set Value to merge field where your record ID is specified (see examples in p.1)
   • Specify Result Handler to work with received data, e.g., some Update Variable action with proper parameters (3)
     
4. Set some event to trigger these actions, e.g., onChange event on lookup field.