go through option 3 family, which has sub-options, so called 3, 3a, 3x. The main differences are how to transmit user plane connectivity between Radio Access Network and Core Network. We can find a bunch of explanation on TR 38.801 and TR 38.804. Split bearer concept has been introduced in Dual Connectivity at TS 37.340. For control plane, EPC is going through MeNB to the UE. For user plane, big difference between 3/3x and 3a is X1-U interface between MeNB and SgNB (3/3x) or none (3a). Option 3 is defined to use an MCG split bearer, which would require more process (investment) on the current eNB side. I am not sure how much operators would like to do with this. Option 3a is done to use a perfect Split bearer, which let operators invest on the gNB that they are going to deploy. Then option 3x is combined and modified practically to use an SCG slit bearer, which let operators save budgets on the eNB side and have benefit in utilizing the user plane through both MeNB and SgNB. Here is the summary on option 3 family below based on TR 38.801 and TR 38.804.