Which part of the body is obligatory to wash sometimes in minor ablution, but not at other times, and how does the ruling concerning its washing change?

The body part that must be washed in minor ritual purity (wudu') at certain times and not others is the feet up to the ankles. Washing them is obligatory if they are uncovered. However, washing them is not obligatory if one is wearing socks or similar coverings; rather, wiping over them takes the place of washing.