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Can you provide more context to what you are looking for?
For PI Tags, you can write annotations to them which can detail why a value is reading abnormal. You can use this function as a way to show maintenance and failure reasonings.
In Asset Framework (AF), you can use event frames to block up time periods where assets are down or in maintenance. You can use attributes in AF that users can input messages.
I'm assuming that you wish to use iMessage application to write to those comments and text data? In that case, you'll need to write a custom AF-SDK application that can parse the message into the appropriate PI Tags in PI Data Archive or Elements in AF.