Master and Secondary key in 5G

Hi 5G experts.
Please tell difference between key to use IE.
I am finding value as master and secondary what does it indicates?

It is basically observed during Split bearers with 5G PDCP and 5G/4G RLC data transfer.

What master and secondary indicates?
What I understand split happen at NR PDCP layer.

AFAIK, there are some user plane encryptions done in Direct bearer and Split bearer.
These keys are related to that Master for MCG (4G) and Secondary for SCG (5G) in NSA network.
Once the PDCP config is done and moved to RLC these keys are configured, and for Split bearers we have key to use as Secondary where 5G PDCP layer will split into 5G RLC/PHY and 4G RLC/PHY.

Ok, so it means master for direct bearer. Seems logical. Thanks.

You may refer this post, it seems it can’t be tagged to split bearer but the PDCP terminating Node, be it split or direct bearer.

5G-NR (EN-DC) Bearer Concept.