[Koha] the definition of AllowOnShelfHolds?

I have a question regarding the option "AllowOnShelfHolds". With the option, we will able to disallow hold requests to be placed on items that are not checked out. But in case of multiple items for one record, the rule seems not work. For example, we have 3 items for one record, one of the 3 items was checkout. User will be able place a hold on the item even 2 of them are not checkouted. Is this correct behavior? Thanks, Justin