This is the most important question you should ask yourself. People tend to use only that one conversion bringing money to their business (purchase or a booking), but this is not the right approach.
When you start with online advertising, you should think about the right strategy to adopt. From how to tease new prospects to get interested in your service or product, to how you can make them book or purchase that later. Depending on where the prospects are in your business discovery, you might want to optimize for other (soft) conversions.
A soft conversion you could consider is ‘sing-ups’. Getting emails from prospects is easier than a final purchase. Another option could be ‘time spent on your site’. These conversions are meant for new prospects and will be cheaper and faster to obtain.
Once you have these valuable prospects, you can then email them or target them with another advertising campaign, this time aiming for final purchases or bookings.
There are many strategies to consider. If you don’t have a lot of experience, we would recommend you to get help from experts.