HP50G - What is "Entry" for?

[Dave Britten]

It cycles the entry mode through normal, algebraic, and programming. In normal mode, commands (such as +) are executed immediately when pressed. In programming mode, they're placed in the current entry line, with spaces as necessary to delimit them from other surrounding commands. In algebraic mode, everything is entered as-is, with no delimiting spaces.

Normal mode is the default mode of operation for doing math on the stack.
Algebraic mode is automatically selected when you press ' and begin entering an algebraic expression.
Programming mode is, naturally, the default mode when editing a UserRPL program.

In my case, the most common use of Entry is to switch from algebraic back to programming while editing a program.

There may be other subtle details and differences between them, but that's the basic idea.