Meaning of LFL and XHDR in Db2

I have seen the following messages in Db2diag.log but I do not know what they are. I cannot find any documentation about LFL or XHDR. I know it is related to HADR and log archiving, but what exactly?

2021-07-20-06.26.20.280271-240 E264696A655          LEVEL: Info
PID     : 6947266              TID : 5757           PROC : db2sysc 0
INSTANCE: db2inst1             NODE : 000           DB   : SAMPLE
HOSTNAME: db2server
EDUID   : 5757                 EDUNAME: db2lfr.0 (SAMPLE) 0
FUNCTION: DB2 UDB, recovery manager, sqlplfrFMReadLog, probe:2825
DATA #1 : SQLPLFR_SCAN_ID, PD_TYPE_SQLPLFR_SCAN_ID, 8 bytes
LFR Scan Num            = 898596933
LFR Scan Caller's EDUID = 20147
MESSAGE : Reached the max number of retries waiting for the LFL in XHDR. Moving
          to next extent.
DATA #2 : SQLPG_EXTENT_NUM, PD_TYPE_SQLPG_EXTENT_NUM, 4 bytes
702955

Do you know one of these terms and where can I get more information?