question about large object

Hi,

I am reading code about large object of pgsql and have a question:

in pg_largeobject.h:

CATALOG(pg_largeobject,2613) BKI_WITHOUT_OIDS
{
Oid loid; /* Identifier of large object */
int4 pageno; /* Page number (starting from 0) */
bytea data; /* Data for page (may be zero-length) */
} FormData_pg_largeobject;

what's mean of pageno? or what 's "page" of a large object refer to?
is this "page"(pageno) refer to "chunk"(chunk number) of lob, as opposed to
real data page? (or just one data page to store one chunk of lob)

Thanks!

Jiong