The Future Lab database engine Bbuuzzb uses a concept called a tid which stands for table ID.

When a table is opened, a tid is returned to the caller. A tid is nothing more than a sequential unsigned integer that the database engine assigns to uniquely identify what is being opened. All high level database functions that do something with table, record or field data require a tid as the first parameter.

Since it is possible and completely normal that a specific table can be opened a multiple number of times even by the same user, the tid is a completely unique way of identifying the specific instance. Each tid has a file pointer and other management variables that are completely unique to that specific instance.