JabRef has following flags for fields to distinguish between to be shown in the BibTeX source tab and to be written to a file.

In net.sf.jabref.gui.BibtexFields this is shown as follows:

private static final int STANDARD = 0x01; // it is a standard bibtex-field
private static final int PRIVATE = 0x02; // internal use, e.g. owner, timestamp
private static final int DISPLAYABLE = 0x04; // These fields cannot be shown inside the source editor panel
private static final int WRITEABLE = 0x08; // These fields will not be saved to the .bib file.

The comment at DISPLAYABLE and WRITEABLE should IMHO be the other way round.

With #391, the source editor tab also shows write-only fields when the entry was not modified:

A possible solution is to rethink non-displayable fields. My current opinion is to write these markings in a separate @comment entry (jabref-meta) or as comment above the entry itself to keep the original entry clean. Reason: (i) For me as user it is not understandable why the displayed rendering is different from the writing in the .bib file. (ii) For me as developer I do not want to offer easy change JabRef's internals. Therefore, I'd like to separate JabRef's markings from the entry data. This should remove the DISPLAYABLE and WRITEABLE flags.

Finally, the flags STANDARD and PRIVATE should be obsolete and removed.