This is the current list of distinct values for @reason (used, as it is, incorrectly, on <supplied>):

• corrected against Bergeron
• EEBO transcription gap. Letter distinguishable by context
• gap in inking; missing letter distinguishable by context
• gap in inking. Letter distinguishable by context
• Facsimile unclear. Addition proofed in David Bergeron's critical edition
• letters missing in the EEBO transcription. Proofed against David Bergeron's critical edition
• EEBO Transcription gap. Missing letters proofed from David Bergeron's critical edition
• new page; catchword euer on previous page
• gap in inking; missing letters obvious from context
• piece of type is a broken semi-colon or an italic colon. This piece of type appears on B2r, but also appears twice on the other side of the sheet, on both B2v and B4v. This mystery requires more investigation.
• justifiable by surrounding context and indicated character spacing
• indicated by catchword on previous page
• checked against Bergeron
• chacked against Bergeron
• gap in inking; text proofed against David Bergeron's text
• gap in inking; letter obvious from context
• gap in inking; missing letters distinguishable by context
• letter distinguishable by context
• gap in inking; text proofed against David M. Bergeron's transcription
• gap in inking. Missing letters obvious from context
• faded ink. Missing letters obvious from context
• faded ink; missing letters obvious from context
• faded ink; missing letter obvious from context
• inking error. Missing letters obvious from context
• Imperfectly inked type; letter obvious from context.
• gap in inking: missing letter obvious from context
• gap in inking: proofed against EBBA facsimile
• page cropped
• page cropped; proofed against HEBE1
• uninked type in Folger Shakespeare Library copy filmed for EEBO
• faded ink: proofed against STC 14685.7
• faded ink: missing letter obvious from context
• verified with Strong's Concordance
• verified with Stong's Concordance
• searched through Thomas Adams sermons in Google Books
• misprint; substitute letter obvious from context
• heavy impression on verso onbscuring the recto print--checked verse with online Latin Bible
• from the Apocryphal book of Esdras, but cannot find the direct source
• should be the equivalent of 'trial,' instead of two words. Obvious from the context
• one word: obvious from the context
• missing letter obvious from context
• faded ink; Biblical citation obvious from the context
• faded ink; missing punctuation obvious from context
• gap in inking; Latin term meaning brought forth or uttered
• letter missing; context obvious
• letter missing
• unclear
• underinking
• scan cut off
• scan cut off; context obvious
• unclear; supplied from mol:STOW1
• unclear; context obvious
• underinking; context obvious
• Letter unclear; context obvious
• character missing
• scan unclear
• bleedthrough
• Page cropped; context obvious
• Letter missing; context obvious
• page cropped; context obvious
• scan unclear; context obvious
• letters missing
• letter unclear; context obvious
• gap in inking; context obvious
• gap in the inking
• overinking
• ink bleedthrough
• letter missing; catchword from preceeding page is fortie
• Page cut off
• gap in inking; missing letter obvious from context

<supplied> appears in Stow 1598, the Mayoral Shows, and Other Texts. It appears 207 times.

@reason is available on four distinct elements: <gap>, <supplied>, <surplus> and <unclear>, with slightly different definitions and different suggested values. One thing we should consider is whether we also need to use @reason on the other elements, and create value lists for them. But in the meantime, this is my suggested list of values to cover the above cases. Note that this excludes information about the source of what is supplied; that should be done with @evidence and @source. We're trying to give the reason that something was or had to be supplied.

Notes:

• "EEBO transcription gap" is not a reason for supplying something. We are not doing an edition of an EEBO/TCP text; we're doing our own edition. If TCP encoders missed something, we should silently correct it. We are only starting from the TCP transcription as a convenience; we do not treat them as an authority, nor do we expect them to be correct.
• "letters missing in the EEBO transcription. Proofed against David Bergeron's critical edition" shows two issues; the first is described above, but the second suggests that we did not even consult the facsimile. The process should be: consult the facsimile; if it's not clear, try to supply something (@reason="unclear"), and then explain the source of the correction using @source and/or @evidence.
• "new page; catchword euer on previous page" presumably means that "euer" is missing from the facsimile (@reason="omitted-in-original") and we restored it based on the catchword (@evidence="internal").

Any more detail should be converted into an editorial note. So the fact that, for instance, we restored a missing word based on a catchword on the preceding page should be explained in a note. All the existing @reason values can be converted into notes automatically.

I suggest that external sources for supplied text always be specified using @source pointing through a mol: URI at their <bibl>; that a <bibl> must be created if the document isn't yet in the bibliography; and that we use the three recommended values for @evidence from the Guidelines:

• internal
• external
• conjecture

although of course we may add to these in the future.