Whether a signal starts something is decided by signal rules — your company's data, under System Setup → CRM → Signal rules, not by settings buried in code. Each rule says: for this kind of signal (optionally, from this source), either start a lead, attach it to the party's history only, or keep it for the measures only.
Two rules are seeded on, following the design your business approved: a store order and an abandoned cart each start a lead — a store order as a Store order lead, a cart as a Website lead. Each carries a minimum value: below it, the signal is still recorded and filed, but it starts nothing. The seeded minimum is "any value" — every order and cart starts a lead until you set a floor of your own. Raise it, switch a rule off, or add rules of your own; signals with no rule covering them are recorded and start nothing.