I have attached the picture. The picture shows two different VDCs in the same Organisation.
OK so i think the story is a vApp can only contain VMs within its VDC
yes, a vApp can only be deployed to a single Organization vDC.
We use the term vDC is a few places, so it's handy to say Organization vDC (aka Org vDC or OvDC) verses Provider (PvDC).
=> Now i assume the abitlity to change is controlled by the "Resource Allocation Tab" ?
Mine says "the VDC in which this VM is deployed does not support controlling resource allocations"
No. This has to do with the type of Organization vDC the VMs are deployed to.
You are either use Allocation Pool or Pay-As-You-Go.
This tab is unlocked when using the Reservation Pool model.