RelatedField RelatedObject
Is the word Related in RelatedField RelatedObject actually referring to the general meaning of the word as opposed to the database meaning of the word related
ie It means the field information related to the column you have just clicked the ellipsis on as opposed to any table / join relationship etc
I suppose you are talking about the IActiveQueryBuilderExpressionBuilderParams.Related* set of properties.
When the OnCustomExpressionBuilder event is fired for condition cell, the expression in condition cell can be incomplete (for example " = 1"). The left part of the expression is stored in the "Expression" cell of the grid (for example "Orders.OrderId"). The complete condition in the SQL is "Orders.OrderID = 1".
For the sample above, the RelatedExpression property contains left part of the splitted condition: "Orders.OrderID", the RelatedObject and RelatedField refer to the corresponding MetadataObject and MetadataField object respectively.