In EN-DC, how SN PDCP decides how much data to forward toward SN RLC and MN RLC?

Hi Experts.
In EN-DC, how SN PDCP decides how much data to forward toward SN RLC and MN RLC?

In UL or DL?

In both, UL and DL.

To my understanding: for UL phone is informed which is primary and secondary path and threshold for dala split.
For DL UE is not informed and is based on NW parameters.
Like buffer occupancy delay, throughput etc.

Yes.
Are there any % kind of parameters like max 20 % buffer can be send toward eNB like that?

I think is vendor dependant. No 3GPP for this.