If I send a proposal, is that spot automatically put on hold?

This article clarifies that sending a proposal does not automatically reserve or hold a spot for your clients. If you want to ensure a spot is reserved while they review your proposal, you should apply an appointment hold separately.

Proposals do not automatically hold spots for your clients. If you wish to block off a spot for a client while they consider your proposal, you'll need to apply an appointment hold.

However, opus1.io now supports a combined proposal-appointment-hold. To learn more about sending proposals, click here. 

Note: Holds for Session based proposals are not currently supported.

To learn more about opus1.io's combined proposal-appointment-hold, click here.