Confusing comment in xlog.c or am I missing something?

I was just going through the xlog.c and I came across following which
confused me:

Given,

src/include/access/xlogdefs.h

#define XLogSegsPerFile (((uint32) 0xffffffff) / XLogSegSize)
#define XLogFileSize (XLogSegsPerFile * XLogSegSize)

Also,
typedef struct XLogRecPtr
{
uint32 xlogid; /* log file #, 0 based */
uint32 xrecoff; /* byte offset of location
in log file */
} XLogRecPtr;

Check for crossing of xlog "segment" boundary?

src/include/access/trasam/xlog.c

/* Check for crossing of xlog segment boundary */
if (RecPtr->xrecoff >= XLogFileSize)
{
(RecPtr->xlogid)++;
RecPtr->xrecoff = 0;
}

Is that xlog "file" boundary or am I missing something?

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