The Custom step is the catch-all. It is a title, a description and an optional file, with no assumptions about what the user has to do. Use it for a requirement that none of the other step types describes, and reach for a purpose-built type whenever one exists.
It is the most flexible step and the least automated. Because Vome does not know what the requirement is, it cannot check it, so a Custom step is essentially an instruction plus a completion record. Typical uses are a police record check the user requests from a local service, a piece of equipment they have to collect, or a rule your organization has that does not exist anywhere else.
Open Sequences in the left-hand navigation and open the sequence you are building.
Click + Add a step.
Choose Custom.
Pick an entry under ADD A PREVIOUSLY SAVED ENTRY, or click New to create one.
Enter the title and description, add a file if the user needs one, then save.
Yes. Turn on Attach a file for users to download, fill out and then upload a completed version, which is the same Supply & Collect option the File Upload step has. The user then gets a download button and an upload button on the step.

The user sees the step title, your description with any links you added, and a download button if you attached a file. If you turned on Supply & Collect they also get an upload button. Otherwise there is nothing to submit.
It depends on the option above:
Without Supply & Collect: the step runs Pending, then Complete. Either you mark it, or the user marks themselves complete with a Mark as complete button, depending on the step's settings.
With Supply & Collect: the step runs Pending, then Uploaded, then Complete, exactly like a File Upload step.
Custom is the right answer surprisingly rarely. Before using it, check whether one of these describes your requirement, because each of them tracks itself:
You need a document back: File Upload.
You need a document signed: Fillable PDF.
You need answers to questions, including consent: Fillable form.
They need to watch something: Video.
They need to complete training: Course or E-Learning.
You need a background check: Background Check.
The step has several parts to tick off: Checklist.
You need to meet them: Interview. There is a session to attend: Orientation.

Custom carries a title, a description and an optional file, for requirements nothing else fits.
Supply & Collect turns it into a two-way file exchange.
Without Supply & Collect, completion is always manual.
With Supply & Collect, it runs Pending, Uploaded, Complete like File Upload.
Prefer a purpose-built step type whenever one describes what you need.
For a side by side view of every step type, see What are the different step categories when adding steps in a sequence?