Freelance GraphicsText interchange
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There are multiple ways to achieve this. You can create a button with state set by Your bool variable and two different captions (A and M) depending on this state. You can toggle visibility of text depending on the variable state (when high - A is visible and M is not, when low - other way around). You may even create a logic in controller to write a string and just show value of this string in graphic.
I recommend toggling the visibility - for this use Graphic Symbol option from Animate menu. You can find more info in Engineering Manual - Operator Station Configuration.
Hope it helps,