When you compose an email, the email addresses are Auto-filled from the email addresses in your Contacts application. In case you are an Organization user, the organization contacts or the Glabal Address list also appears in the Auto fill for you to choose and include the email addresses in your email.
If you have Categories added in the Contacts section, you can type in the category name and the category will be available for Autofill. Once chosen, the contact email addresses in the particular category will automatically be inserted in the To/ Cc/ Bcc address bar, in which you are typing in.
You can also click on the Address book icon in the To/ Cc/ Bcc boxes to view the Address book dialog. You can choose the the email addresses, which are alphabetically sorted in the Address Book. The address book has a separate section called Categories, from which you can insert email address based on Categories.