understand clog fetch

Question

https://speakerdeck.com/macdice/improving-the-slru-subsystem?slide=29
contact author, get the C function source code.

CREATE FUNCTION test_clog_fetch(x1 pg_catalog.xid,
                                x2 pg_catalog.xid,
                                loops pg_catalog.int4)
    RETURNS pg_catalog.void STRICT
    AS 'slrubench','test_clog_fetch' LANGUAGE C;

#include "postgres.h"

#include "fmgr.h"
#include "access/clog.h"
#include "access/xlogdefs.h"
#include "miscadmin.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(test_clog_fetch);

Datum
test_clog_fetch(PG_FUNCTION_ARGS)
{
    TransactionId x1 = PG_GETARG_TRANSACTIONID(0);
    TransactionId x2 = PG_GETARG_TRANSACTIONID(1);
    int loops = PG_GETARG_INT32(2);

    elog(NOTICE, "xid range %u, %u; loop = %d", x1, x2, loops);
    for (int i = 0; i < loops; ++i)
    {
        for (TransactionId xid = x1; xid < x2; ++xid)
        {
            XLogRecPtr lsn;
            TransactionIdGetStatus(xid, &lsn);
        }
        CHECK_FOR_INTERRUPTS();
    }

    PG_RETURN_VOID();
}

sql part:

SELECT pg_current_xact_id();
create table test as select * from public.tenk1;
SELECT pg_current_xact_id();

select test_clog_fetch('807'::xid,'809'::xid,1e4::int);
 test_clog_fetch
-----------------

(1 row)

Time: 10.003 ms

regression=# select name,blks_zeroed,blks_hit,blks_read,blks_written,blks_exists,flushes,truncates
from pg_stat_slru;
      name       | blks_zeroed | blks_hit | blks_read | blks_written | blks_exists | flushes | truncates
-----------------+-------------+----------+-----------+--------------+-------------+---------+-----------
 CommitTs        |           0 |        0 |         0 |            0 |           0 |       7 |         0
 MultiXactMember |           1 |        0 |         0 |            1 |           0 |       7 |         0
 MultiXactOffset |           1 |        1 |         3 |            4 |           0 |       7 |         0
 Notify          |           0 |        0 |         0 |            0 |           0 |       0 |         0
 Serial          |           0 |        0 |         0 |            0 |           0 |       0 |         0
 Subtrans        |           6 |        0 |         0 |            4 |           0 |       7 |         7
 Xact            |           2 |    23971 |         3 |            6 |           0 |       7 |         0
 other           |           0 |        0 |         0 |            0 |           0 |       0 |         0
(8 rows)

I want to understand what does 23971 mean. overall I can understand input output of function TransactionIdGetStatus. but lsn part I am not famiilar with.

between xid 807, xid 809. I created a table, table relpages is 345, reltuples is 10000.