This read-only value reports the number of memory blocks currently in use on a given module, where a block is a unit of memory that can be allocated or freed; the description notes it can be used to check for leaks by noting the value, performing and then undoing an operation, and confirming the value returns to its starting point. Admins use it as a memory-leak diagnostic tool, especially useful when the byte-level free/in-use figures alone don't clearly reveal a slow leak. An admin suspecting a leak in a specific feature would note this value, exercise that feature repeatedly, then check whether the block count returns to its original level afterward.