This option will initially mark all subscribers that need to receive the campaign as “todo” in the table and then start processing on all the “todo” entries.

It may save a little on SQL time when you have a large amount of subscribers, as the query to search for subscribers is run fewer times.

Comparison:

When MESSAGEQUEUE_PREPARE is 0, each batch of messages is processed as follows:

1. Find all subscribers who need to receive the campaign and haven't received it yet
2. Dispatch the campaign to some subscribers (batch size settings determine how many)

When MESSAGEQUEUE_PREPARE is 1, the first batch of messages is processed as follows:

1. Find all subscribers who need to receive the campaign (as in step 1 above)
2. Mark the found subscribers as “todo” in the database
3. Send some of the marked subscribers the campaign

Then for all subsequent batches:

1. Find subscribers marked “todo”
2. Send some of them the campaign (batch size settings determine how many)

See also MAX_SENDPROCESSES