Append from < TableName > always looks for the table TableName.dbf on the disk regardless if it's open or not already.

>Why does a
>APPEND FROM a FOR a.fieldx='F' append the whole table while

It checks the value of fieldx in the current record of 'a' only. If current record in 'a' has a.fieldx='F' the whole table will be appended. If a.fieldx <> 'F' than nothing will be appended

>APPEND FROM a FOR fieldx='F' appends only selected records?

In this case every record is checked for fieldx='F' before it's appended.