The FILEBUFFER is written to the record, FP points to.

Mode and submode select the file to use. Filepointer FP must be a valid filepointer from previous calls to FILENEXT or FILEPREV.
If FP equals to -1 then a new record is written.

After the writing FP may have a changed value, but points to the written record.

After writing a new record all previously determined filepointers become invalid.