9.1 64bit on AIX.

I read the manual but still unclear about ASLHEAPSZ - what it's used for and how to set/tune it. Can you suggest?

according the doc
The application support layer heap represents a communication buffer between the local application and its associated agent. This buffer is allocated as shared memory by each database manager agent that is started.

Recommendation: If your application's requests are generally small and the application is running on a memory constrained system, you might want to reduce the value of this parameter. If your queries are generally very large, requiring more than one send and receive request, and your system is not constrained by memory, you might want to increase the value of this parameter.