There is no difference.
The only thing matters here is , on which table you are creating a layout.
If layout is on TableA , then
relationship will help to relate to TableB based on same Site_id.
If layout is on Tableb , then
relationship will help to relate to TableA based on same Site_id.
Exactly what Saigopal Das said. The relationship is exactly the same.
However, in terms of FileMaker they also include a relationship graph that have Tutorial: What are Table Occurrences?, which are based on tables but are only representations of the tables. Usually those are taken in consideration from left to right for different things, but for the most part it doesn't matter.
All true, but storage, validation and auto-enter field options can change the "left to right" significance and how the relationship functions. One of those ID fields might auto-enter a serial number, making this a one to many relationship with the serial number field's table being the "one" side of the relationship.
If id has global storage or is defined as an unstored calculation in one of the two such fields, this can also greatly affect how the relationship works.