As the Help describes, the complete surface (all addressable sectors) divided to 10000 blocks, so each blocks represent 1/10000 of the complete disk capacity. During the surface test, you can see the size of each blocks (see the upper right corner).
For example, if you have 1 TB hard disk, then each blocks = 100 MB.
If you have a 300 GB partition -> then you can specify start block = 0, last block = 2999 to test the area used by that partition.
And if you have a 2nd partition on the rest, then you can specify start block = 3000, last block = 9999 to test the area used by that partition.
(or you may adjust the values slightly, eg. test 0 - 3049 and 2950 - 9999)
As the surface test performs at low level (sector-level), it is not possible to specify logical drive (and especially not cylinders, heads, sectors - as these values are not valid for modern disk drives). But this is not required at all, as you can specify part of the drive to be tested with this option.
This is why it is possible to test even disks which contain no partition(s) or partition(s) formatted under Linux/MAC which may be not readable under Windows (but the drive can be tested, diagnosed, repaired by Hard Disk Sentinel).
Moreover, this way all sectors outside the partitions (eg. partition table/MBR, hidden sectors, etc.) are tested properly.