Explicit(明确的\I清楚的） Messaging is when a server sends information to a client on a specific request. Explicit Messaging uses TCP/IP and requires that the memory location of the information to be sent to the client be defined in the instruction itself.

Implicit（含蓄的） Messaging is when a server sends information from predefined memory locations to a client at a given interval. For PLCs, this given interval is the RPI cycle time. Implicit Messaging uses UDP and is faster than Explicit Messaging and is thus intended for time-critical applications.

Unconnected Explicit Requests and Responses – used mainly for establishing explicit and I/O connections, but can also be used for one time requests to attributes of data model objects.

Explicit Messaging (Class 3) connections – are strictly point-to-point connections. The connections cyclically query the adapter for some data or cyclically write data to the adapter. Transmitted using TCP.

I/O Messaging (Class 1) connections – can be point-to-point or multicast. Used for frequent exchange of process data,Transmitted using UDP.

Connection limits:

• Class 3 connections – there can be maximum six Class 3 connections simultaneously.

• Class 1 connection – there can be only one Class 1 connection simultaneously.

Link:https://library.automationdirect.com/ethernetip-implicit-vs-explicit-messaging/