The Fillable form step puts one of your Vome forms inside a sequence. The user answers it without leaving the platform, the step completes on its own when they submit, and every answer comes back as data you can search, filter, report on and map onto profile fields.
It is a pointer to a form that already exists in your Forms module. The step does not build the form, it selects one. The same form can be used by more than one step and by more than one sequence.
Open Sequences in the left-hand navigation and open the sequence you are building.
Click + Add a step.
Choose Fillable forms.
Under Select a form in the Forms module, search for the form you want and select it. Click Refresh List if you have only just created it, or New Form to build one now.
Give the step a title, then save.

The user sees the step title, your description, and a Complete form button that opens the form inside Vome. Required questions have to be answered before the form will submit.
The user is Pending until they submit. Submitting moves them to Submitted, and you mark the step Complete once you have reviewed the answers.
If you do not need to review anything, the step can skip the Submitted status and complete automatically the moment the form is submitted. That setting lives in the step's own settings.
Into the Forms module, alongside every other submission of that form, and the person's progress stays visible from the sequence. Because the answers are real fields rather than a file, you can search and filter on them, pull them into a report, and map them onto profile fields so the profile updates itself.
Use Fillable PDF when the output has to be a document: a waiver that must keep its exact wording and layout, or anything that needs a signature in a particular place. Use Fillable form when what you actually want is the answers.
Yes, and it is the recommended way. Build a form with a Digital Consent element, then point a Fillable form step at it. See How would I gather digital consent using the Sequences feature?
The step selects an existing form from your Forms module rather than creating one.
Set a sequence-only form to Private.
Statuses run Pending, then Submitted, then Complete, and Submitted can be skipped.
Answers land in the Forms module as data you can search, report on and map to profiles.
Choose Fillable PDF instead when you need a signed document rather than answers.
For a side by side view of every step type, see What are the different step categories when adding steps in a sequence?