Customer properties

In the documentation for the Syncro API, one of the properties in the Customer object that /Customers returns is a hash labeled “properties”

“properties”: {},

But there is nothing mentioned in the documentation about what this hash contains.
Can the official API documentation please be updated to reflect the exact purpose of this hash in the API?

My investigations thus far…
Retrieving all my customers with

string customers = await response.Content.ReadAsStringAsync();

I find these 4 properties.
“properties”:{“Website URL”:"",“notification_billing”:“0”,“notification_reports”:“0”,“notification_marketing”:“0”}

However, if I add a custom field for customers that is called Status and put Active in that custom field then the properties hash contains the following

Notice that the Key called “Website URL” has disappeared.

So it appears that the properties hash is used to provide a way to obtain the custom fields for customers.
However, the keys “notification_billing”,“notification_reports”,“notification_marketing”
are not custom fields. They are in the customer settings section.

I’m assuming the API has evolved overtime to use the properties hash to display custom fields in addition to these three notification check flags.
But why does “Website URL” disappear, and where is this field in the Syncro WebUI?