Message text is a bit tricky and limited.
The "message text" blocks must be connected to an "open connector" block to be able to be downloaded to the LOGO (in simulation this is not necessary) otherwise the error: "Blocks without connection to an output" appears in the info window
Not all block types appear in the list to be selected for message text (as In and Outputs, Flags, Simple gates). However many of those can be selected via the ON/OFF icon. But just a maximum of 8 messages of those kind can be displayed. The ON/OFF test occupies 8 characters on the display (not just 3).
Special flags exist as "Flag M29" that when it is 1 the back light turns red.
Acknowledge Messages are messages that stay as long in the display until a user resets them.