This one recently caught me out when creating a custom user profile property and thought I'd share this.
To create a custom user profile property navigate to the the User Profile Service Application –> Manage User Profile Properties and click on New Property.
On the New Property page where you enter all the configuration of the property type right at the bottom there is a ‘Add New Mapping’ section which is where you would sync and connect this to your data connection (noting that my data connection had been removed from view below).
The requirement for my property was simply to be a string value with no connection to a data source and the above section confused me that I needed to add a property regardless. This is not the case and you do not have to add a property at this point for the creation to be successful.
Should you need to add a property mapping to a data source above shows how it looks (again my data connections have been removed).
There are many different mappings that can be configured for creating these properties however my post was simply to make you aware of what caught me out.