PTF SO08257 introduced the DFLTSIZ= parm. It may be confusing to understand exactly what this parm does, and when it should be specified. This article addresses that.
Release : 19.0
Component : CA IDMS
The new DFLTSIZ= parameter is part of the support that SO08257 introduces for dynamic screen size. 3270 terminal model emulations that have more rows and columns than the 24x80 Mod2, like the 43x80 Mod4 and 27x132 Mod5 have the ability to display the smaller Mod2 size. The larger size is the 'alternate' size, the smaller size is the 'default', or 'primary' size.
Some sites like to set their emulations to a non-standard 3270 alternate display size, like 60x132. This is called an IBM Dynamic size. While some systems tolerate this, IDMS, prior to the PTF, would force one to start a new emulator session with a supported 3270 model screen size as the alternate size. Having to start a new session was deemed a pain point.
What the PTF does is allow emulations that are non-standard 3270 alternate display sizes to access IDMS DC without starting a new session. Any non-standard alternate size is ignored, and the default size is utilized. Telling IDMS what the default size is, is not necessary for VTAM access. For UCF, both TSO and CICS tend to insist the default size is a Mod2, when it may actually be a Mod3 or Mod4. This UCF disposition led to the introduction of the parameter DFLTSIZE= to tell us what default screen size to actually use, so we could support a Mod3 and Mod4 as the default size, and not just the Mod2 that TSO and CICS would try to implement.
If a site's emulations are using a standard supported 3270 Model type, no action is necessary, and there is no impact on an existing system. The DFLTSIZE parameter only means something if a non-standard (dynamic) alternate screen size is detected. If a dynamic alternate size is detected, and there's no DFLTSIZ parameter, a 3270 Mod2 screen size is assumed as the default.
More details about supporting alternate screen sizes can be foudn here: