There are several MX platforms such MX5, MX80, MX240, MX480, MX960, MX2008, MX2010, MX2020, etc.
All consist of different H/w components (some may have similar modules/unit) and varies from platform to platform and have different framework. Example of Basic H/w for MX 960 platform consist of Switch Control Board(SCB), Routing-Engine (RE), Forwarding Plane or known Flexible PIC Concentrator (FPC), Chassis Fan System, etc, etc.
Flexible PIC Concentrators (FPCs) and Physical Interface Cards (PICs) function similarly to MICs and MPCs. PICs provide physical interfaces for the router and install into FPCs which provide packet forwarding services. Each FPC supports up to two Physical Interface Cards (PICs). You can install PICs of different media types on the same router as long as the router supports those PICs. PFE (Packet Forwarding Engine) is part of FPC that is responsible for Packet forwards, Number of PFE varies from FPC variants.