How does Engage deal with multi-wave journeys / sequences?

A Journey can exist of multiple Sends (= a sequence). For example, once somebody joins the journey, they receive one email today, one other email in 3 days from now, and one sms in 7 days from now.

To support this kind of sequences, there are 2 approaches:

1. Apply saturation control to every separate Send
In case the messages in each journey can exist on their own, it may make sense to apply saturation control to each Send. This means that Engage could decide to exclude contacts from some of the messages in the sequence. Hence, there is no guarantee contacts will receive all messages from the sequence.
To do this: add a DESelect Engage decision split in front of every send activity in your journey. Link each activity to a different Send without send date in Engage. You could decide to name your Sends as '<campaign name> <send 1 name>', <campaign name> <send 2 name>' etc. For the example from above, names could be: 'Welcome journey email 1', 'Welcome journey email 2', 'Welcome journey sms 1'.
Engage will then evaluate for each of the Sends if the contact can be included when the contact reaches the Engage journey activity.

2. Apply saturation control to the sequence as a whole

When the different messages in a journey belong together, you'll want to make sure that everybody who enters the journey receives all messages in the journey.
To do so, add a DESelect Engage decision split only in front of the first send activity in your journey.
Engage will then evaluate a contact before the first Send, depending on the availability of slots for a contact to receive that first message. Once a contact is included for that first Send, the remaining messages on the journey are sent without the intervention of Engage. This also means that it's possible contacts will actually get oversaturated because of the following messages, as those are no longer evaluated against a contact's saturation levels.