Pointers to multiple index allocation mappings
sysindexes points only at first IAM page in IAM pages chain. IAM pages chain is a doubly linked list - ie. each page in chain (obviously except first and last ones) has pointers to the next page and to previous page in its header.

Look here: Managing Space Used by Objects


  An IAM page covers a 4-GB range in a file and is the same coverage as
  a GAM or SGAM page. If the allocation unit contains extents from more
  than one file, or more than one 4-GB range of a file, there will be
  multiple IAM pages linked in an IAM chain. Therefore, each allocation
  unit has at least one IAM page for each file on which it has extents.
  There may also be more than one IAM page on a file, if the range of
  the extents on the file allocated to the allocation unit exceeds the
  range that a single IAM page can record.