If you want to mark whether a booking enquiry has come from an email or over the phone or via a third party source you can create Sources to mark the enquiry/booking with.
Go to Settings
> Venue Group
> Booking Sources
> Add Another
> Add "Over The Phone" and "Email" as two new Booking Sources (you can also add any third parties you work with)
When you add the enquiry to Collins, in the customers booking details screen, the "Source" drop down box will pull through all the sources you have added in your admin, with which you can tag your bookings.
In your data download you will see a "Source" column if you want to analyse your sources of bookings