When you create a new person contact, you have the option to add a preferred name. The preferred name can also be added or edited on the person's details tab.

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As well as keeping a record of what your clients prefer to be called, this field is a useful way to personalize your automatic client task emails. Default client task emails and reminder emails will include a placeholder for the client's preferred name. If the client does not have a preferred name specified, their first name will be used instead. 

If you don't add a preferred name while creating the contact, by default it will be the first name of the contact. This can be updated within the details tab of an already created contact, open up the 'Details' Tab' and click edit. There you will see the option to update the preferred name.