This implementation of ISAM uses shared memory, and it has reached the maximum number of concurrent users for which the shared memory was configured.
The word users can be misleading; the limit is on the number of concurrent application programs using the database server. It is possible for one user to start multiple applications at the same time. For example, when a user starts the IBM Informix 4GL Programmer's Environment, it opens a session with the database server. When that user issues a command to compile a 4GL program, the 4GL compiler starts and also opens a session with the database server. During a compile, this user has two sessions running.