Reading through the mi append file right now and I fail to understand the difference between the two commands. Could someone please explain the difference to me.

The file says

mi append is logically equivalent to append; see [D] append. The resulting data have M = max(Mmaster, Musing), not their sum
.

What does this mean? I have 2 waves of data that I combined using append (quite new to stata and this is my first go at trying to do an empirical work) not knowing that mi append exists. In what ways have I messed up the data by using append vs mi append?

Overall I am trying to use the command mi xtset but failing horribly, so I am trying to work my way backwards and find what I have messed up. Thanks for any help!