A command as simple as:

Select TABLE_A
Append from TABLE_B FOR FIELDX=.T.

it appends all records, not only FIELDX=.T.
FIELDX in TABLE_B is a logical field.

I tried also

Append from TABLE_B FOR TABLE_B.FIELDX=.T.

Same result.

Why things apparently so simple are sometimes so complicated?

TIA for your time.

Ulises