EventPV is a PLC variable, that defines whether the line is executed or not. If left empty the line will be always executed.
LocalPV is the variable where the value to write or read is located.
In you example the first line writes 3 registers to station number 1 with the value from variable LocalPV4 to offset 0 (HoldingRegister 0) using Modbus Function 16(Preset multiple registers) when EventPV4 is set to true.
The second line reads 3 holding registers from station number 1 starting from offset 0 (HoldingRegister 0) using Modbus Function 3 (Read holding registers) when EventPV3 is set to true.
EventPV
EventPV = 0 Entry not processed (skipped)
EventPV = 1 Entry processed
If a response frame is obtained without an error, the associated EventPV is set to 0 once. If an EventPV is not specified, then this entry must be executed cyclically.The BOOL, SINT, and USINT variable types are supported as Event PV, with no restriction on entry length, and dynamic PVs (important: indicated by *) are also supported. If no EventPV is specified, this entry must be executed cyclically.
Station number (destination address) (USINT)
This entry defines the destination address of the slave station. Valid destination addresses are between 0 and 247, with 0 acting as a broadcast (message to all stations).DataPV
PV that is read from or written to depending on the specified function code. Variable types BOOL, SINT and USINT are supported for Modbus functions 1, 2, 5 and 15; variable types UINT and INT are supported for functions 3, 4, 6 and 16. Any length of the entry is possible; dynamic PVs can also be used (important: specify with *).When naming PVs, the guidelines listed under Names of process variables and Notes for using application modules must be observed.
Offset (UINT)
For the offset of Modbus variables, see the documentation for the Modbus slave.Length (UINT)
Number of coils or registers