In Rel10, what are those 4 bits in format1b used for?

Hi All.
I have a question.

In release 10, format1b is said to have 4 bit harq.
What are those bits used for?

UE needs to report HARQ feedback for each TB.
In case of carrier aggregation (2 CC in this case), UE may get upto 4TBs (2 TB on primary cell and 2 more on secondary cell) in a TTI.
So, UE need 4 bits to acknowledge these 4 TBs.

Thanks! But what about id it is TDD set up single cell?
These 4 bits have any use there?

In case of TDD, for some DL/UL configs, we have less UL slots than DL slots.
Those cases we need to send HARQ feedback of multiple DL slots in a single UL slot.
There UE will use concepts like HARQ bundling/multiplexing to send feedback of for multiple DL slots.
These cases it needs more bits.

Oh I get it… Thank you​:pray::pray::pray::pray: so much sir!