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?