When creating a custom form you will see a toggle button labeled "Internal Use", see below.
The difference between the two is very important to understand and is described in detail below.
External forms are forms that can be shared with individuals outside your Lawmatics platform, i.e. leads, clients, contacts, etc. There are many ways to share forms such as via automations, email, or simply copying the link to the form and sharing it. The bottom line is that anyone with the link to an external form can access, fill and submit that form.
Internal forms are different in several ways, but the most important is that they CANNOT be shared with individuals outside of your Lawmatics platform. This means that not only can the form only be opened by users of Lawmatics but the user also must be signed into Lawmatics in order to open, fill and submit the form.
Internal forms also give you access to certain fields that you do not have access to in an external form. For example, fields such as practice area, source, campaign, matter owner are fields that are to be used internally only. They are not available in external forms in an effort to protect the possibility of exposing data that shouldn't be exposed.