What is the difference between Physical cell id and cell id in LTE?

Question

For what purpose Cell ID can be used?

Answer 1

Physical cell id (PCI) is used for physical layer procedure. UE uses it to decode PSS and SSS. PCI range from 0 to 503. This is similar to scrambling code in UMTS
physical CellID=PSS+3*SSS. As PSS is between 0 to 2 and SSS 0 to 167
PCI is trasmitted in MIB

Ex: 2014 Dec 24 15:29:46.026 [00] 0xB0C1 LTE RRC MIB Message Log Packet
Version = 1
Physical cell ID = 45
FREQ = 2130
SFN = 568
Number of TX Antennas = 2
DL Bandwidth = 10 MHz (50)

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Cell ID uniquely identifies eNB within network. UE uses this for identifying a cell whereas core network uses this for OAM.
It is transmitted in SIB1
e.g
message c1 : systemInformationBlockType1 :
{
cellIdentity '00000000 00000000 11100101 1010'B,
}

Answer 2

PCI can be re-used within the network or across the network but cell id uniquely identity a cell within the network that's why Global cell identity consists of plmn + cell Id . PCI ranges only from 0 to 503.