The basket manager shows all baskets (ie., document sets) within the Migrate system and is available for accounts on dedicated portals. If you have been assigned an account on this shared portal a basket should have been setup for you. Send us an email if you cannot select a basket in the wizard pages.
You “carry” a basket throughout the migration process and apply migration steps to the documents in it. At any time you can take documents out of the basket or add new ones to it. If you purchase a subscription you will get a portal where you can create as many baskets as you need to migrate different sets of documents.