Since AutoCAD LT 2008, you've been able to include multiline attributes in blocks, giving you more flexibility in entering & displaying information.
But when you go to edit the value of your multiline attribute, some things might appear to be missing:
Not a lot of options there. And sometimes, that might be okay. You don't always *need* fancy formatting in an attribute. But if you want the full power of mtext in your multiline attribute, simply set ATTIPE (as in ATTribute In-Place Editor) to 1.
ATTIPE is set to 0 by default to help maintain maximum compatibility with previous AutoCAD LT versions. But depending on who you're sending files to, there's nothing to stop you from turning the full editor on.