The epoch is for the position given in fields 4-5 and 10-15. The field is blank for epoch J2000.0.
[]Epoch J2000.0 was chosen for the Input Catalogue publication, corresponding to the `standard' epoch common to all recently-published catalogues, even though the epoch for the version used for satellite operations was 1990.
If the epoch is specified, this indicates that only a single
epoch position is available, and therefore no proper motion can be
derived. Where relevant, the stated epoch refers to the average of the
epochs for 
and 
.