what's the difference between to_lsn and last_lsn in xtrabackup_checkpoints file?

[apple0315]

Does anyone know clearly what’s the difference between to_lsn and last_lsn in xtrabackup_checkpoints file?
Sometimes they are the same. Sometimes not.

[angry_RMB]

yean ，I get the same problem， I want now the reason and the backup will still be useful？