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 :)