In the relationship graph, a table occurrence (TO) is an instance of a base table. In this sense, a TO is similar to a symbolic link or an alias in an operating system file system. There may be one, none or hundreds* of TOs referring to the same base table.
Although every TO of a base table refers to the same table, and the same set of records, it creates a different context from which they can be referenced.
The PTO (primary table occurrence) is the table occurrence that has the lowest index of all the TO of a base table. Unless one think of changing it, it is the one that is specified by default in the [calculation window].
Fun fact: a TO is the only object that is able to reference a base table. All other objects in FileMaker must reference a TO.
* There is limit on the number of TOs that refer to the same base table in the v12 file format, except file size limits (8TB) or available disk space. There is a limit of one million base tables. FMPv15 Tech Specs