The Landsat 7 thermal band 6 data files refer to the gain settings of which the band is acquired. Since band 6 is acquired in both high and low gain, the gain settings are provided as two separate band files:
FILE_NAME_BAND_6_VCID_1 (band 61 or 6L) provides an expanded dynamic range and lower radiometric resolution (sensitivity), with less saturation at high Digital Number (DN) values.
FILE_NAME_BAND_6_VCID_2 (band 62 or 6H) has higher radiometric resolution (sensitivity), although it has a more restricted dynamic range.
Additional details about Band 6 gain states can be found in the Landsat 7 Data Users Handbook.