Melange is a feature in ProfilePress that is capable of converting any (combination of) login, registration password reset and even edit profile form template to a working WordPress equivalent.
Examples of what can be built with the melange feature is a login, registration and password reset form widget; multi-step registration forms e.t.c.
This multi-step registration form is an example of what melange is capable of.
Building say a login form with melange is similar to the normal way. The only difference is, while the latter do not require a container shortcode, the former do which is
[pp-edit-profile-form] for login, registration, password reset and edit profile form.
Take a look at the Lucid Account Tab Widget theme included in ProfilePress to see a live example of how melange works.