Identifiers help to define the various applications, networks and drivers to which security rules apply. They are necessary when you create security rules, and must be created beforehand.
Each identifier consists of an unlimited number of entries linked by a logical “OR” operator, i.e., a security rule applies as soon as at least one of the identifier’s entries is recognized.
Identifier entries make it possible to group various resources under the same identifier in order to pool any rules that may concern these resources.
There is no difference between creating two identifiers with single entry each and a single identifier containing two entries if all identifiers are associated with the same rule.