Publication codes

Just as a note, the following publications codes are/were used (as far as I know):

• G - e.g. G2/88 - for decisions (and opinions) of the Enlarged Board of Appeal on referrals
• R - e.g. R4/18 -  for decisions of the Enlarged Board of Appeal on petitions for review
• J - e.g. J1/18 - for decisions of the Legal Board of Appeal
• T - e.g. T3/16 - for decisions of the Technical Boards of Appeal
• W - e.g. W2/10 - for decisions of a Technical Board of Appeal in a PCT protest procedure (concerning lack of unity of a PCT application in the international phase). The PCT protest procedure no longer involves the Boards of appeal, but the substantive case law on unity of invention remains relevant.
• D - e.g. D1/79 - for decisions of the Disciplinary Board of Appeal (both on EQE matters and disciplinary cases).

Furthermore, these codes actually refer to cases. Hence, the codes can also be used to refer to pending proceedings (e.g. G1/18) (but also simply for parties in pending appeal procedures) and can also be used to refer cases terminated without a decision (e.g. G2/14, OJ 2015 A13).

Publication codes have also been used for the following publications, which are not decisions of the Boards of appeal:

• L - e.g. L15/05 (OJ 2005 p.357) - These were publications in the OJ titled "Legal advice from the European Patent Office". All Legal Advices were abolished by Dec. Pres. OJ 2012 p.446 (which gives a non-exhaustive list of the Legal Advices).  The Legal Advice documents are not included in the EPO case law database.
• V - e.g. V1/84 (OJ 1984 p.117). Some decisions of the Examining Divisions and Opposition Divisions were published in the OJ. Some of these published first instance decisions were given such a publication code, though not in the OJ but in some books. The reason is that these decisions were included in the EPO's internal case law database ‘Mimosa’ with such number, the letter ‘V’ stands for ‘Vorinstanz’ as explained in Singer/Stauder/Luginbuhl EPU 8th ed. Art.129 rdn. 9 fn.2. See also Watchorn&Veronese, PCT Procedures, 2016, p. 408; see V8/94, OJ 1995 p.388 in Vissers' AEPC, edition 2018, Art.52(2):2). There seems to be neither a public search function nor a list for this kind of publications, so the OJ reference is necessary to refer to them. 
• DB - e.g. DB2/92 published in OJ 1994 p.750 for a decision of the Disciplinary Board of the EPO (i.e the Disciplinary Board, not the Disciplinary Board of Appeal). Another example is DB 03/15 in OJ 2019 A40.
• CD - e.g. CD 07/2012 for a decision of a Disciplinary Committee of epi published in EPI Information 2017/4. 

The numbering is usually consecutive, but there are some exceptions.

There are a few J decision in the "900 series"; these concern decisions which have been published only in redacted form because they are excluded from file inspection. They deal with legal incapacity of a professional representative and the resulting interruption of proceedings (J900/85, J901/86, J902/87) or (the denied) legal incapacity of the applicant acting pro se (J903/87).

J900/85 is published in redacted form in OJ 1985 p.159 without a case number and the case is sometimes referred to as JXX/XX. The case number J900/85 was added as a fictitious number in the case law database (see here).

J901/86 was published as J../86 in OJ 1987 p.528

J902/87 was published as J../87 in OJ 1988 p.323

J903/87 as published as J../87  in OJ 1988 p.177

Decision G2301/15, G2302/15 and G2301/16 concern proceedings for removal from office of a Board member under Art. 23 EPC.

In addition, in the case of interlocutory decisions, these are given the same number as the main decision.
There appears to be no settled usage for differentiating between decisions in the same case. I have used a letter on this weblog (e.g. T 1627b/09). Visser uses e.g. G2/08a and G2/08b.
The most reliable and unambiguous identification of a decision is with the ECLI number (e.g. ECLI:EP:BA:2018:T193414.20181008); the ECLI is a case number developed in EU context (wiki). The number can be resolved e.g. as https://e-justice.europa.eu/ecli/ECLI:EP:BA:2018:T193414.20181008.
Three decisions or even more in total are also possible, e.g. T83/05 of 22.05.2007 (leading to G 2/07), of 08.07.2013 leading to G2/13 and finally of 10.09.2015 (order to maintain the patent in amended form); similarly T 1242/06.
ECLI numbers can be used to unambiguously identify not only interlocutory decisions but also in case of ancillary decisions (i.e. after the final decision) e.g. in T 1934/14 of  08.10.2018 (ECLI:EP:BA:2018:T193414.20181008).

An excellent explanation is also given on wikipedia (link)

In addition, the Boards give decisions a distribution key (see here):

A - publication in OJ

B - distributed to "board chairmen and members"

C - distributed to  "board chairmen"

D - no distribution