What is a difference between cross-lingual IR and multi-lingual IR? In papers usually Cross-Lingual Information Retrieval (CLIR) and Multi-Lingual Information Retrieval (MLIR) use equivalently or distinctly. I want to know is there any difference between these two terms?
In this paper they differentiate between these two terms as follows:


  While CLIR is concerned with retrieval for given language pairs (i.e.
  all the documents are given in a specific language and need to be
  retrieved to queries in another language), MLIR is concerned with
  retrieval from a document collection where documents in multiple
  languages co-exist and need to be retrieved to a query in any
  language. MLIR is thus inherently more difficult than CLIR.