The object which describes a WAL range always has three keys:
TimelineThe timeline for this range of WAL records, as an integer.
Start-LSNThe LSN at which replay must begin on the indicated timeline in order to make use of this backup. The LSN is stored in the format normally used by PostgreSQL; that is, it is a string consisting of two strings of hexadecimal characters, each with a length of between 1 and 8, separated by a slash.
End-LSN
The earliest LSN at which replay on the indicated timeline may end when
making use of this backup. This is stored in the same format as
Start-LSN.
Ordinarily, there will be only a single WAL range. However, if a backup is taken from a standby which switches timelines during the backup due to an upstream promotion, it is possible for multiple ranges to be present, each with a different timeline. There will never be multiple WAL ranges present for the same timeline.