Table relations will usually fill themselves in, but they are the variables you use to connect A table to B table. Most tables it will be the Company, and one of the Primary Keys for that table, for example Part’s primary keys are Company and PartNum. Your query is joining the EntityGLC and Vendor tables, checking the Key1 field against the VendorNum casted as an integer, and then if they match it’s returning both, and if they don’t it’s still returning the EntityGLC row without the Vendor data per the join type.