Perhaps the HP-42S was to be released as a replacement for the aging HP-41 series as it is designed to be compatible with all programs written for the HP-41.

The 42S, however, has a much smaller form factor than the 41, and features many more built-in functions, such as a matrix editor, complex number support, an equation solver, user-defined menus, and basic graphing capabilities (the 42S can draw graphs only by programs).

Additionally, it features a two-line dot matrix display, which made stack manipulation easier to understand.

[1] The HP-42S is keystroke-programmable, meaning that it can remember and later execute sequences of keystrokes to solve particular problems of interest to the user.

[4] Functionality Programmability Guides and Collections of Keystroke Programs Documentation for the HP 42S calculator at hpcalc.org[5][6]