The EnableCompletionRule command turns on the specified completion rule(s) in the Typesetting package for use in 2-D input, while the DisableCompletionRule command turns off the specified completion rule(s) in the Typesetting package for use in 2-D input.

There are too many rules to be able to list them all here, but all are available in the Rules area of the interactive Typesetting Assistant.

As a simple example, if the rule $''BesselJ''$ was enabled for completion, one could enter "BesselJ" within the 2-D editor, then hit the completion key (Esc), and one of the presented options will be a formatted BesselJ function (specifically a function $J$ with a symbolic subscript $v$ as a function of a variable $x$, where the $v$ and $x$ are entry points).

It should be noted that functions entered via completion parse in the expected way even if the corresponding parse rule is disabled.