What is object Server?
Actually, you question has less to do with "DB2" than it does with one of IBM's main products - IBM i on Power (also known as System i, iSeries, and AS/400 in the past). For whatever reason, IBM chooses to refer to a lot of things as "Objects" within their system. This gets very confusing to hear, because in and of itself, the operating system i5/OS is not particularly object oriented, nor is the programming language (RPG) that comes with the system. That being said, RPG is starting to gain some object-oriented functionality within the last few releases of the language.

The key point of the link you shared is that the Object Server is the piece of the iSeries that allows end users access to reports, etc. (Strangely, a lot of these reports, end-programs, etc. are referred to as objects as mentioned above). Hence the term Object Server.

Now, all this being said, the iSeries does come with a version of DB2. It is called DB2 for i and it is actually integrated into the operating system and the file system. It differs from DB2 LUW (Linux, Unix, Windows) in that it relies on an EBCDIC representation of text rather than ASCII. It also functions a bit different under the covers. Because it is integrated into the OS, the OS takes care of monitoring DB2, so you can actually get away with having no DBA on staff. We did it for years. I've heard similar of other iSeries shops. (And since IBM created the RPG language for the iSeries it integrates incredibly well with DB2 as well.)

Now if it also helps, a physical iSeries server can be divided into several LPARs (logical partitions). Perhaps a "virtual server" if you will. So an LPAR is an "instance" of the iSeries.

Since DB2 for i is integrated into the OS, there is one instance of DB2 per instance of the OS. In other words, each LPAR is an instance of DB2 (and also like THE one and only database at the same time). And unlike DB2 LUW, you cannot have more than one instance on the same server. And lastly, a "library" for DB2 for i, is the equivalent of a schema, and a "file" for DB2 for i, is the equivalent of a table.

Hope this helps clarify your thoughts.