How do I properly use multimarker in an xml file?

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I am trying to detect three AR trackers with the ar_track_alvar package link ar_track_alvar package. The three trackers should be detected as a bundle. My xml file which describes the markers looks as follows:

<multimarker markers="3">

<marker index="0" status="1">
  <corner x="-3.45" y="-3.45" z="0" />
  <corner x="3.45" y="-3.45" z="0" />
  <corner x="3.45" y="3.45" z="0" />
  <corner x="-3.45" y="3.45" z="0" />
</marker>

<marker index="1" status="1">
  <corner x="-3.45" y="-16.85" z="0" />
  <corner x="3.45" y="-16.85" z="0" />
  <corner x="3.45" y="-9.95" z="0" />
  <corner x="-3.45" y="-9.95" z="0" />
</marker>

<marker index="2" status="1">
  <corner x="21.15" y="-16.85" z="0" />
  <corner x="28.05" y="-16.85" z="0" />
  <corner x="28.05" y="-9.95" z="0" />
  <corner x="21.15" y="-9.95" z="0" />
</marker>

</multimarker>

• As far as I understood the first marker has to have index 0. For the other markers the index does not matter, right?
• What does the status mean? What arguments are valid?
• Sometimes I get the following error: ERROR InferCorners: "ar_marker_0" passed to lookupTransform argument source_frame does not exist. Sometimes it is also another marker number. How can I avoid this error or what is the reason for it?