These properties for a subform container are meant to describe the links between the one table (master) and the many table (subform). Access will automatically fill these properties in for you IF you have properly described the table relationships. To do that merely use the tools/relationships menu to add tables to the builder and describe the relationships between the tables. You can accomplish this only if the Primary Key/Foreign Keys are identical data types. After describing these relationships you should rebuild the subform container. The LinkMasterField will contain the Primary Key from the master table, e.g. UserID, and the LinkChildField will be filled in with the Foreign Key in the many table, e.g. UserID.
You can also do these manually but I recommend the longer and better method.
Steve King