Hi,
An instance, consists of set of Virtual process (VPs), shared memory and disk components configured to accept and serve the user requests. Such a collection of components is called a database server instance.
The environment variable $INFORMIXSERVER points to an active instance. Such an instance has it's own $ONCONFIG file, a separate information line in sqlhosts file and other associated configuration files.
When multiple database server instances and their associated shared memory and disk structures coexist on a single server machine, it is called multiple residency.
Regards,
Shriyan
"The learning and knowledge that we have, is, at the most, but little compared with that of which we are ignorant."