Táve supports two types of client-facing URLs for your proposals, invoices, contact forms, etc:

Secure Domain: example.tave.com
Custom Domain: clients.example.com 

Both of these options can be edited in the brand editor by going to Settings and finding the Brands link on the left navigation. I've underlined the part you provide.

To set up the secure domain, all you have to do is enter one in the Secure Domain field. There are a few restrictions, such as it can't be a dictionary word, misleading, or already in use.

Whatever you pick will be turned into a subdomain of "tave.com", so my "Karen Lisa Artistic Photography" brand uses "karenlisa.tave.com", which I set up by entering karenlisa into the Secure Domain field. After saving, your links will all use this new branded tave.com subdomain.

A custom domain is completely branded as you point a subdomain of your website to Táve, instead of the other way around. For instance I configured clients.karenlisa.com for my "Karen Lisa Artistic Photography" brand, making the "clients" subdomain point to my Táve brand.  Configuring this can get complicated, so it's best to review our how to set up a CNAME record tutorial.

It definitely looks more permanent and professional to have your name as the secure domain.

Important: if you change your secure domain after sending out client links, you will have to update your client with the new links as the URL will have changed.

If I use a Custom Domain, will my Secure Domain ever be used?

Yes, quite possibly!

Your custom domain can't be encrypted, so if your client needs to enter in a credit card, the page will automatically change to the secure domain (thus its name!). The only difference between the two pages would be the URL in the address bar and the browser's lock icon next to it... your page branding will be identical and otherwise indistinguishable. 

If you aren't accepting credit cards on your Táve site, your custom domain will always be used, but it's best to set a secure domain now so you have one set up in case that changes in the future and so you've reserved the one you want.