Allen-Bradley 1756-L61 ControlLogix 5561 Programming Manual page 30

Chapter 2 Organizing Tags
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Avoid using the same name for both a controller tag and a program tag. Within a
program, you cannot reference a controller tag if a tag of the same name exists as a
program tag for that program.
Certain tags must be controller scope (controller tag).
Table 3 - Controller Scope Tags
If you want to use the tag
In more than one program in the project
In a Message (MSG) instruction
To produce or consume data
In any of the seven AXIS data types
To communicate with a PanelView terminal
None of the above
Rockwell Automation Publication 1756-PM004D-EN-P - September 2012
Then assign this scope
Controller scope (controller tags)
Program scope (program tags)