The standard definitions:
Simultaneous submission means sending a submission to multiple agents or editors, while multiple submission means sending different submissions to a single agent or editor.
If someone says simultaneous is okay, that is not a red flag.
Most allow simultaneous but dislike multiple. Some are the opposite--Glimmer Train, for example. The really snotty ones don't like either.
The usage is the same for queries and submissions. Quoting
Duotrope:
Simultaneous submissions This refers to the practice of sending a single story to different publications at the same time. When simultaneous submissions are allowed, most editors prefer for you to let them know up front, and always let them know if the piece is accepted elsewhere. When simultaneous submissions aren't allowed, some editors blacklist writers who get caught sending them simultaneous submissions.
Multiple submissions When more than one of your stories or poems are under consideration with a single publication, you have sent multiple submissions. Very few fiction publications allow multiple submissions, and when they do, be sure to follow their guidelines. Many impose length restrictions on multiple submissions. Most publications prefer for you to wait until you hear back from them on a pending submission before submitting another.