More basic questions about Spartan 2 IOB

Still trying to understand the very basic IOB in all its details...

In the Spartan 2 datasheet I read:
'The IOB includes an optional register in the output path,
the input path, and the 3 state control pin.

In the diagram this is clear.

Questions arise:
Can the 3 state register be controlled independently?
Are all 3 registers (plus the delay) active by default?
Peeking in various .ucf files I find that some properties can be set:

NET my_net Clkp                         Does this mean positive clock on
                                        register?
NET my_net  OFFSET = OUT : 2.5  2.5 nS  Does this mean delay in output?
Is there also a programmable output delay?

So what are the exact commands for the UCF file to:
Switch off / on the input register.
Select a pos or neg clock for the input and output registers.
What are the defaults?

Is there a specific xapp I should (have) read that describes this
in detail, preferably with human readable examples?

The IOB diagram is clear enough by itself, but many things remain a mystery
to me.
For example if using clocked IOB on input, WHERE does the clock come from?
I have 3 clock pins in use, how do you select?
Is this hardwired somehow?

Sorry now I am getting into detail, but this I need to know to make any
sensible circuits...