I am not sure I understand your question.

The Library Dot Product (dotp) in Skyline is calculated by comparing the chromatogram peak areas in the Skyline document with the intensities in the spectrum in the spectrum library.
The spectrum that the chromatogram peak areas are being compared against is the spectrum that you see in the "View > Library Match" window.
(that menu item has moved to "View > Libraries > Library Match" in Skyline-daily)
-- Nick

if there are two MS2 spectra in library, which one it be used? Thanks,

Two or more spectra from same peptide in the library. Thanks,

The spectrum that had the best score from the peptide search engine will be used.

When you create a spectral library from peptide search results, first a file is created whose name ends in ".redundant.blib".
Then, a program called "BlibFilter.exe" looks at all of the spectra for each peptide & charge state and chooses the one with the best score from the search engine. That creates a file whose name ends in ".blib" and it will have at most one spectrum per peptide sequence and charge state.
If you have unchecked the "Keep redundant library" checkbox then the ".redundant.blib" file will be deleted.
-- Nick