What is a 'unit' and what is 'nesting'?

[DougBeney]

size_t unit;	/* reallocation unit size (0 = read-only buffer) */

Line #34 buffer.h

My guess is that it is used as a unit to incrementally grow the size of your buffer when needed.

Why is a 'unit' required and what are some good practices to know what number to set?"

I noticed that the wiki uses "16" as a unit and hoedown.c uses these two different values for the input/output buffer sizes:

#define DEF_IUNIT 1024
#define DEF_OUNIT 64

Thanks!

EDIT:
Also, what is 'nesting' in the context of hoedown_html_renderer_new and hoedown-document_new? "Maximum level of block nesting parsed." Is it just about markdown lists?