In case a PSU-ID allows more than one kind of data item – e.g. IBAN, Name, email address – to identify a PSU, the PSU-ID-Type specifies the type of used data item, hence, whether it’s a name, an IBAN or an email address.
The ASPSP can mandate the usage of a PSU-ID Type. Compare also, IO16.
Note: Also in the case where the ASPSP supports different types of PSU-Ids he might not support the PSU-Id-Type in the requests, since he is able to distinguish between the different typs whithout this explicit information, e.g. he might recognize whether an IBAN or an email adress is entered.