Here is the official answer from Sun Developer Expert Assistance. I happened to ask this questions some days before. Somebody should add this to the GF wiki:
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AS 9.0 --> Glassfish v1
AS 9.1 --> Glassfish v2
AS 9.1 UR1 (aka AS 9.1.0_01) --> Glassfish v2 UR1
AS 9.1 UR2 (aka AS 9.1.0_02) --> Glassfish v2 UR2
AS 9.1 UR2 p06 (aka Sun Glassfish Enterprise Server v2.1) --> Glassfish v2.1
The name changed came in AS 9.1 UR2 p06, SunJava System App Server was rebranded as Sun Glassfish Enterprise Server. From that point on, Glassfish open source and Glassfish (Sun's) commercial version are equivalent.
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