It's more complicated than that. The "Submit" button is for the draft - response - draft type assignment where a student submits a file, the teacher reviews it and reverts the assignment to "draft," the student revises and clicks "submit," and the cycle goes on until the teacher finally grades it. You get this button in the Submission *settings* section of Assignment settings > Require students click submit button set to Yes.
In submission *types,* if you click "file submission" and DO NOT click "Require students click submit button" in Submission settings, your assignment will end up with a button labeled "Add submission." This is a one time submission of one or more files. This cannot be used for the draft-respond-draft cycle in the 1st paragraph above.
This is Moodle 2.3+ versions.
Mary will correct me if I'm wrong