Normal variables, constants or remanent variables that are known throughout the project can be declared as global variables.
In a project you can define a local variable which has the same name as a global variable. In this case within a POU the locally defined variable will be used.
It is not allowed to name two global variables identically. For example, you will get a compiler error if you have defined a variable "var1" in the PLC Configuration as well as in a global variables list.