Radial buttons are located next to a contacts profile details i.e. Voice (phone), Email, Mobile etc - see example below shown in red: 

How do Radial buttons work?

1. Clicking on a Radial button will set up the preferred contact number default or email address if a contact has more than one email.  The radial button is useful if there are several contact numbers for one contact.  

The example above shows the default radial button applied to Voice (this is Phone line).

2.  If you have a number of mobile numbers then select the default to the mobile you prefer to be contacted on.

3.  It will apply the same with email address too - if you have multiple email addresses but want to use one in particular to receive you email calendar event or task reminders then use the radial button to set which email address will be the default one.

What happens if I do not set up a Radial button?

Then the system will automatically set up the first phone number, email address, mobile number listed as the Default.