What is difference between conditional and optional IE's in LTE?

Question

Uplink NAS transport message is used between eNodeB and MME to pass NAS messages between UE and MME transparently.
Since eNodeB has already communicated TAI and E-CGI as part of "Initial UE message" and MME knows current serving (cell and tai ) of UE then why both IEs are mandatory in Uplink NAS transport. I think it should be optional and it should be included in the message when there is a change in serving (cell or tai or both).

Answer 1

Not just LTE but true for any technology -

Optional IE - user/network/operator is free to include or exclude based on in its requirement.

Conditional IE - Conditional IE is dependent on some condition and based on the condition it should be included or not included. Let me put the generalized example - support you have a HSS/HLR data and in some message you need to include the user profile based on if a home user or not. So in this case if the information like is_homeuser is true then you must include the user profile (this is a hypothetical example)

Comment if not clear :)