Sparse Memory

configname: CONFIG_SPARSEMEM_MANUAL

Memory model
└─>Sparse Memory
In linux kernel since version 2.6.13 (release Date: 2005-08-28)  
This will be the only option for some systems, including
memory hot-plug systems. This is normal.

This option provides efficient support for systems with
holes is their physical address space and allows memory
hot-plug and hot-remove.

If unsure, choose "Flat Memory" over this option.