explaining FilePut method

That's right, Tamas.

Here's a VB example I found on MSDN which makes the use of -1 clear though for FileGet rather than FilePut:

Thanks Volpes!
If I understood it correctly, the third parameter refers to whether the next record should be written or shouldn't. If the param is -1 or void, the next record shall be written. 

The third parameter, RecordNumber, to the FilePut method is optional and has a default value of -1.

If you pass -1, or omit it altogether, this means that the next record or byte after the last FileGet or FilePut function, or pointed to by the last Seek function, will be written to.