About self-joining relationships
A self-join is a relationship in which both match fields are defined in the same table. Define a self-join to create relationships between records in the same table. Use a self-join in a portal on a layout of the current table to display a subset of data that's in the current table, such as all the employees of each manager.
When you define a self-joining relationship in the relationships graph, FileMaker Pro generates a second occurrence of the table upon which you are basing the self-join. FileMaker Pro does this to prevent the relationships from forming a cycle, which would make it impossible to evaluate.
A self-joining relationship looks like this:
Two occurences of the same table with a line between fields showing a self-join
This relationship returns related records in TableG when the value in TextFieldG1 is the same as the value in TextFieldG2.
Related topics 
About single-criteria relationships
About multi-criteria relationships
About relationships using comparative operators
About relationships that return a range of records