-

Safety Semantic Warning: The global variable '...' is used within a single POU. Consider to use a local variable.

The variable mentioned in the error message is declared with 'Global' declaration keyword in the Data List of the Safety PLC and as External in a POU. Such resource-global variables can be used in any POU of the project.
This variable, however, is only used in one POU. If no additional use in any other POU is intended, declare the variable as local variable by
  1. setting 'Usage = Local' in the POU's variable table
  2. and deleting the 'Global' declaration in the Data List.

This way, it is prevented that the variable is used unintentionally in the project.

Note
Restrict the number of global variables to the minimum requirement.