At Clustdoc, a client portal is a public access URL you can put on your website, social media, email signatures to allow anyone to start a process without you having to do anything.
If you want, it's also possible to disable the registration on your portal, let's see how:
Go to Resources>Client Portals
Select the portal you're thinking of or create a new portal link
Head over to the Configuration>General tab and untick the Allow new applications option