If a constant is set to public (meaning it can be edited by the user from the front end) then it can only be used once in a Menta application. Options > Constants Table will show you which constants are set to public. Any attempt to use them more than once will result in the following error:
Public constant used more than once:
Typically a public constant is being used because the simplicity of a simple block is desired. A delay block may be used on three different fan commands and the user desires all of them to have the same delay attribute.
To use one value for multiple simple blocks, the functionality of those simple blocks will have to be recreated using other Menta blocks until a single PVR can be sent to multiple HFBs. For example, instead of a delay block, some configuration of trigger and pulse blocks could delay the output and a PVR could be used to set the length of the delay.
Note: Non-public constants can be used multiple times in simple block parameters and expressions. For example, "pi" could be defined as 3.1415926535 and used in multiple equations. It's just public constants that cannot.