According to the link below, some will grow little bulbs, as well as making topsets, that will fall over, and root themselves...making for a perennial onion bed.

I've also heard them referred to as winter onions, as they can be harvested anytime the ground is not frozen.

Here is a link that might be useful: Top-set onions, tree onions, or walking onions

Regular bulbing onions should be started from seed or transplants. Walking onions are usually used as green onions but you can use the dry bulbs somewhat like shallots.