See answer to other question you just asked.
The system <b>doesn’t</b> handle expanding subfile size. Your program has to handle it, by adding more records, by redisplaying the existing subfile or by doing whatever else you program it to do. The system only allocates additional blocks of memory to hold the new subfile records that you WRITE; if you don’t write any, then the system doesn’t do anything about it.
When a user attempts to scroll past the last record of the subfile that you built, the system will return control to your program. Your program then must decide how to “handle” things.