NSSF - an 5G network function to support the network slicing

3gpp UE Identities

IMSI # MSISDN # External Identifier # IP address # IoT # 5G # 6G

IMSI # (15 Digits/ E.164/ )

The IMSI number is subscriber identification with given operator/ network (PLMN) with a maximum length of 15 digits. The IMSI is consists of an MCC (Mobile Country Code), an MNC (Mobile Network Code), and an MSIN (Mobile Subscriber Identity) 

formation IMSI (15) = MCC (3) + MNC (2-3) + MSIN (9-10)

format - TBCD format

MSISDN format (15 digits/ E.164/ TBCD Format)

The ITU-T recommendation E.164 limits the maximum length of an MSISDN to 15 digits. wherein 1-3 digits are reserved for country code. Prefixes are not included (e.g., 00 prefixes an international MSISDN when dialing from Sweden).

http://www.3gpp.org/wiki/index.php?title=Current%20Capacity%20of%20IMSI%2C%20MSISDN%2C%20and%20IPv6%20Identifiers
https://www.linkedin.com/pulse/mobile-numbering-basics-explained-imsi-vs-msisdn-atte-miettinen/

From 3GPP TS 23.003: 

IMSI: No more than 15 digits. Is composed of three fields: The Mobile Country Code (MCC), the Mobile Network Code (MNC) and the Mobile Subscriber Identification Number (MSIN). The MCC is 3 digits, the MNC is 2 or 3 digits, leaving 9 or 10 digits for the MSIN. This means an operator is alloted between 1 and 10 billion MSIN. This means that between 70 and 700 MCC and MNC values would be needed.
Since the MSIN+MCC together a 12 digit value, any country could be assigned the maximum value.
MSISDN : No more than 15 digits. The Country Code is 1 to 3 digits, and the National Destination Code (NDC) and Subscriber Number (SN) together are 12 to 14 digits. So again, the MSISDN could contain the 12 digits needed for MTC.

There are cases where the diameter AVP can considered to carry either IMSI/ MSISDN. 
User configure, type of Routing Entity type - IMSI, IMPI, MSISDN, IMPU and AVP - will selection of decoding formula of value as MSISDN or IMSI
valid pair (IMSI, IMPI) and (MSISDN, IMPU)