Connection Pool is a very interesting concept in OBIEE. If you are from Discoverer background then you never had to deal with it at the Oracle Discoverer Administrator level. There are java connection pool settings which is different and application server connection pools but these are the settings which are configured by the DBAs.
Even in OBIEE, it is recommended that DBAs have their input in deciding the connection pool settings
The connection pool is an object in the Physical layer that describes access to the data source. It contains information about the connection between the Oracle BI Server and that data source.
The Physical layer in the Administration Tool contains at least one connection pool for each database. When you create the physical layer by importing a schema for a data source, the connection pool is created automatically. You can configure multiple connection pools for a database. Connection pools allow multiple concurrent data source requests (queries) to share a single database connection, reducing the overhead of connecting to a database.
For each connection pool, you must specify the maximum number of concurrent connections allowed. After this limit is reached, the Oracle BI Server routes all other connection requests to another connection pool or, if no other connection pools exist, the connection request waits until a connection becomes available.
Increasing the allowed number of concurrent connections can potentially increase the load on the underlying database accessed by the connection pool. Test and consult with your DBA to make sure the data source can handle the number of connections specified in the connection pool. Also, if the data sources have a charge back system based on the number of connections, you might want to limit the number of concurrent connections to keep the charge-back costs down.
In addition to the potential load and costs associated with the database resources, the Oracle BI Server allocates shared memory for each connection upon server startup. This raises the number of connections and increases Oracle BI Server memory usage