As I know PScell is used in two connectivity.
1. Rel12 Dual Connectivity-- in which UE is connected to one MeNB and one SeNB.
MCG (Master Cell Group): a group of serving cells associated with the MeNB(master) comprising of the PCell and optionally one or more SCells.
SCG (Secondary Cell Group): a group of serving cells associated with the SeNB.comprising of PSCell and optionally one or more Scells.At least one cell in SCG has a configured UL CC and one of them, named PSCell, is configured with PUCCH resources.
When SCG is configured, there is always at least one SCG bearer or one Split bearer.
PSCell cannot be de-activated.
2. Dual Connectivity between LTE and NR- Either master node is LTE/NR & secondary cell group LTE/NR depends on configuration used.
If LTE is master & NR is secondary- In this UE acquires, at least, radio frame timing and SFN of SCG from the NR-PSS/SSS and PBCH of NR PSCell.
If NR is master & LTE is secondary- In this case, the UE acquires radio frame timing and SFN of SCG from PSS/SSS and MIB on LTE PSCell.