Question about the official doc and CLUSTER section : "but you can do this one child table at a time"

[achix]

Dear friends, I am referring to this section in the docs : https://github.com/pgpartman/pg_partman/blob/development/doc/pg_partman_howto.md#partitioning-an-existing-table , where it says :

Just note that this does take an access exclusive lock on the table, but you can do this one child table at a time.

And I am wondering which child table does the doc talk about? Still the doc itself assumes only a single source table. Anyone could shed some light on what is the meaning of this phrase ? Thank you!