EDIT: I believe I tried to remain a bit too abstract in my answer, I've updated my answer to more acutely answer your questions.
I am struggling with the concept of 'Fabric Plane' and 'Switch Fabric'? I haven't found any clear cut difference between them?
At a very high level, SCBs contain Switch Fabrics, and Switch Fabrics contain Fabric Planes. The collective role of these components is to provide connectivity between all of the PFEs on each FPC installed in the chassis.
Switch Fabric: Switch Fabrics physical chips that are contained within the SCBs. Each SCB will have 2 Switch Fabrics.
Fabric Planes: Fabric Planes are more dynamic in nature, the number of Fabirc Planes per Switch Fabric will vary depending upon the MX chassis model.
Why fabric planes are different in MX960 compared with MX240 and MX480.
As previously stated, the role of the SCBs is to provide connectivity between the PFEs on the FPCs. This connectivity, must be organized into a full mesh, this means that each FPC will have the same amount of throughput capacity to every other FPC. Every time you add another FPC, the SCBs must be able to support that full mesh requirement.
Using the book's example, MX240s and MX480s have 2 SCB slots, which will result in 4 Switch Fabrics and 8 Fabric Planes. But, they also have a smaller number of available FPC slots. It simply comes down to the fact that the MX240 and MX480 only need half of the Fabric Planes in order to accomodate the full mesh requirement, leaving the remaining half to be used as spares (backup).
NOTE: To answer the comment you left on Benjamin's answer, yes, those grey boxes are the spare planes in that particular example, be aware, it can vary per FPC type.
If we look at the book's example of the MX960, even though it has an additional SCB slot, it also has more FPC slots (and therefore more PFEs). In order to maintain that full mesh requirement for the increased number of PFEs, half of the Fabric Planes would not be enough. So, it will "partition" the Fabric Planes a bit differently. Now, we end up with 6 total Fabric Planes (3 SCBs, with 2 Switch Fabrics each, and 1 Fabric Plane per Switch Fabric).
How SCB effect throughput?
There are additional complexities to complement what I mentioned above. First, is the fact that different model SCBs support different levels of maximum bandwidth. Second, FPCs also support varying levels of bandwidth. Finally, there are your redundancy and throughput requirements.
Using an MX960 as an example, utilizing fabric redundancy will keep 1 SCB as a hot standby in case that one of the others fail. This means that only 2 SCBs of capacity will ever be used at any one time. You have to account for your network capacity in that those 2 SCBs must be enough to handle the traffic on your router. On the other hand, if you would rather used "increased-bandwidth" mode, you can do so, which will enable all 3 SCBs to be utilized. However, if a SCB fails during a period of high network traffic, you may experience network congestion and performance impact.
MX Series Routers Fabric Resiliency
All of these factors must be considered when deciding what hardware to use.
One final note, there is a newer "2nd Edition" of the MX book that you may find helpful as well.
