This deprecated setting manually forces a port's Ethernet MAU into master or slave clocking role, which only matters when auto-negotiation is turned off, since normally the two ends of a link negotiate this automatically. An administrator would have used this on older gigabit-copper links where auto-negotiation was disabled and the timing-clock roles needed to be set explicitly to get the link to come up at all. With the default value of slave, a port configured this way (with auto-negotiation off) would operate as the timing slave unless explicitly set to master.