I'll give it a try (in a technically simplified way): The HoBT level (HoBT = Heap or B-Tree, see http://technet.microsoft.com/en-us/library/ms189849.aspx) is the structure underlying a table. If the table is not partitioned, you could say HoBT(a) = table(a).

For a partitioned table, each partition is one HoBT, or HoBT(a1) = p(1)table(a), HoBT(a2) = p(2)table(a), etc.

So locking just one HoBT of a partitioned table = locking the partition, but not the entire table.

Or as BOL puts it: A HoBT-level lock does not necessarily lock the aligned HoBTs for the partition.

Personally I'd rephrase that to: A HoBT-level lock does not necessarily lock all aligned HoBTs for an entire partitioned table.

Regards,

Michael