The cursor.explain() somtimes shows:

...
{
  "cursor" : "BtreeCursor _key1_1_key2_1 multi",
...

The "multi" attribute at the end of the index doesn't seem to be documented. It refers to multiple ranges being used to scan the _key1_1_key2_1 index.

For example, if the query condition includes "{key1:{$ne:5}}," the query optimizer might choose to search the index for two ranges:

  "indexBounds" : {
    "key1" : [
      [
        {
          "$minElement" : 1
        },
        5
      ],
      [
        5,
        {
          "$maxElement" : 1
        }
      ]
    ],