ensureguarantees that at least the specified instance(s) exist(s). If they do, there are no further actions taken. If they don't, the system will create what is missing.
ensure 2 Person, AccountHolder
If the case already has two persons that are accountholders, this expression will change nothing.
However, if there is only one person who is accountholder, the system will create a second person with the role AccountHolder.
Older versions of RuLa included the operator
create. It is possible you encounter it in rules that were written before the current version of RuLa. However, going forward, use