What is the difference between bulk SMS and transactional SMS?

Technically they travel the same way. In everything else they differ.

Bulk SMS

One message to many people at once, at a moment you choose. An offer, an invitation, an announcement. You need consent and an opt-out has to be included.

Transactional SMS

One message to one person, triggered by something that person did. An order, a booking, a login attempt. No marketing consent needed, because it is not advertising.

Why the distinction matters

First legally: the rules differ, and an offer at the bottom of a shipping notice turns that message into advertising.

Second operationally: with bulk, a delay of a few minutes is fine; with a verification code it is not. Anyone sending both over the same channel and the same sender risks a large campaign delaying their own codes.

In practice

Separate them. A separate sender, and if the volume is large, a separate route. Then a campaign that lands badly cannot drag your critical messages down with it.