Contains annotations of a peak.
The mz and intensity values contain the same information as a spectrum would have about the peaks, and can be used to map the additional information to the correct peak or reconstruct the annotated spectrum. Additionally the charge of the peak and an arbitrary string annotation can be stored.
The string annotation can be e.g. a fragment type like "y3". This information can be used e.g. to label peaks in TOPPView.
The specific application in OpenProXL uses a more complex syntax to define the larger number of different ion types found in XL-MS data.
In the example "[alpha|ci$y3-H2O-NH3]" "alpha" or "beta" determines on which of the two peptides the fragmentation occurred, "ci" or "xi" determines whether the cross-link and with it the other peptide is contained in the fragment, and the last part is the ion type with the fragmentation position (index) and losses. The separators "|" and "$" are used to separate the parts easily when parsing the annotation.
1.9.1 
