The values used in the FILTCODE column(s) depend on the particular data-logging system. They can even be the standard names of the bands. Thus, FILTCODE may just be a duplicate of the BAND column. These columns are 8-byte character variables, and are normally displayed with A10 format.
Some multichannel instruments have separate filter mechanisms for each detector. For example, a two-channel instrument might use ``red'' and ``blue'' beams. Then the filters used in the ``red'' beam would not affect measurements made in the ``blue'' beam. In this case, the word ``any'' can be inserted in the filter code for the beam that is not used with a given output channel (see examples below).
For spectrometric multichannel instruments, there may be no choice of filters for any channel: the data from different channels are distinguished by position rather than by code in the raw data. Then NFILTCAR can be set to zero, and the FILTCODE column can be omitted from the instrument description file. In this case, the separate channels must still be identified on separate rows in the final data table file (see section I.6, ``Observational data'') by the standard names of the bands.
On the other hand, the Danish 6-channel spectrometer at La Silla has two separate mechanisms for inserting neutral-density filters. In this case, one must append an ND code to the passband name in the data file (see section I.6, ``Observational data''), to indicate the combination of neutral filters in use.
Note that the OPTI-n keyword in the ESO Archive  includes other optical elements than filters, and so is not uniquely related to the numbering of filter wheels.