Student or Learner
I would like to know what does reference mean in this excerpt, when is mentions that "references such as..... cannot be matched". Does it refer to entries? Meaning words you type in a database to look for something?
While the database contains the complete contents of all the major journals and many minor ones as well, its coverage of references and citations is much less complete. References in and citations of articles in the major journals are fairly complete, but references such as ‘private communication’, ‘in press’ or ‘in preparation’ cannot be matched, and author errors in reference listings also introduce potential errors. Astronomical papers may cite and be cited by articles in journals which fall outside the scope of ADS, such as chemistry, maths or biology journals.
A reference is an acknowledgment of another scholar's prior work that is less than a citation. A citation refers to a specific paper in a specific academic journal. The examples of references in your quotation show why they are less than a citation: for example, the person referred to communicated privately with the author, or his or her paper has not yet been published. Nevertheless, academic honesty demands that the prior work of others be acknowledged.