No Each object is main. Material, Plant Vendor Stock etc.
Blocks are subdividing the data of Objects.
If you're transferring Materials alone.
10000 Materials (Object) are Subdivided into 10 blocks (1000 each), these 10 blocks will be transferred in parallel based on the config and resource availability.
Same applies with other objects as well.
No of objects selected for transfer depends upon the scope of the data how its interconnected to other objects.
Go through the link which i've sent, explore you'll get good knowledge
Regards
Vinoth