What is the difference between the Estimate and Order for Service?

The Estimates are for before the move is Booked, i.e. CLOSED-WON. Sometimes this is enough of an agreement, or it could be a much "softer agreement". So, that's where Order for Service and Change Order are useful. They are for confirming the agreement in its legal form.

Keep in mind that, from an Operational perspective, the main result of Booking, on a "softer agreement" or otherwise, sets resources aside on the day(s) scheduled for the move, at the moment it's booked. And so this also leads to the topic of deposits.

The document type of Invoice is processed with an exclusive set of Merge Fields (and different access through Billing and Actuals). The document type of Others is intended for E-Sign capable documents of your choosing.