In both places, “Master” means that the router is the owner of the WiFi SSID. I.e., the devices are connected to the SSID provided by the router.
If the router is acting as a WiFi client, i.e., it connects to an SSID by another router, then the connection will be displayed as “Client”. But this scenario never happens to the pcWRT router. The pcWRT router always acts as a “Master”.