What does the e3 or e4 printed on an IC mean?

So after staring at a few ICs today, I was wondering what the "e3" or "e4" stands for on the top of an IC. For example, here is an e3:

• interesting -- i've never noticed this! – Jason S Sep 13 '11 at 13:52

It is regarding the lead content or plating finish. Here is an excerpt from IPC/JEDEC J-STD-609:

e1 - SnAgCu (shall not be included in category e2)

e2 - Sn alloys with no Bi or Zn excluding SnAgCu

e3 - Sn

e4 - Preplated (e.g., Ag, Au, NiPd, NiPdAu)

e5 - SnZn, SnZnx (no Bi)

e6 - contains Bi

e7 - low temperature solder ($\le$ 150C) containing Indium (no Bi)

e8, e9 - unassigned

• Oh wow, excellent. Thank you for that great answer. John. – user1307 Apr 29 '10 at 8:32