As the PMKcaching, two options in the setting, standard or opmk. What is the definition of these two options?

Standard: indicates PMK Caching: This means that the 802.1x authentication can be skipped on an access point that a client has already authenticated to once before. Only the 4-way handshake needs to happen. This is useful for a client that needs to reconnect to an access point that it roamed away from previously, due to signal loss etc. However, if a client has not roamed to a particular access point during its current working session, it must then authenticate to that specific access point using 802.1x. PMK Caching is the method defined in the 802.11x (WPA/WPA2) specification. Opportunistic Key Caching: With this method, a client device can skip the 802.1x authentication with an access point after a full authentication,and only needs to perform the 4 way handshake when roaming to access points that are centrally managed by the same WLC in an LWAPP or other controller-based infrastructure. This means that the client doesn't need to authenticate with access points that it wants to roam to, as long as the client has authenticated successfully to at least one of the access points in the same zone as the access point that handled the previous successful authentication. In this case, the PMK identifier has been cached at a central location, like the WLC (or wireless switch.) With OKC, the client must support this method for it to be used, even if the infrastructure has been configured with OKC enabled.