Direct Answer: A request is an invitation for your partner to change their behavior, which they are 100% free to accept or decline. A boundary is a statement of what you will do to protect your own well-being if an issue persists. A request depends on their cooperation; a boundary depends entirely on your own follow-through.
When we make a request, we are asking for cooperation. This is a vital and healthy part of any collaborative partnership. However, if our partner says 'No' to a request, we cannot force them without becoming controlling. That is where boundaries come in. A boundary is your own back-up plan.