Landsat 7 acquires thermal data in two bands from one detector in both high (Band 6H) and low (Band 6L) gain. The difference in gain settings is important to different types of studies (i.e., clouds vs. deserts). Aside from the resolution differences and saturation on the high and low ends, the two bands provide the same Digital Numbers (DN) for every pixel.
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, and is more likely to saturate over hot targets.