For loans, how do I set the U.S. Rule?

On the interest change event, the interest method can be set to either actuarial or simple interest. By setting simple interest, any interest that is not satisfied for a period will go into a separate bucket where further interest is NOT charged upon this unsatisfied amount. On a subsequent principal change event (i.e., payment), any unsatisfied interest is accounted for first unless the principal first option has been set.

How do I create an HP 12c compatible cashflow?

On the interest change event of the default loan template, the round balance option is none and the round decimal digits is 2. This means that no rounding is performed at each intermediate step of the cashflow. For display purposes, the amounts are subject to bankers rounding down to the decimal digits set by the locale.

To create an HP 12c compatible cashflow on the interest change event, set the round balance option to bankers and the round decimal digits to 0. The means that bankers rounding will be performed at each intermediate step of the cashflow and the amounts rounded up or down to eliminate any fractional part.

How do extend the AmFn Wasm UI for my own locale, template groups, etc.?

Creating another locale for a particular spoken language and geographical region is a more challenging endeavour that ultimately posts the new locale's files to the GitHub repository for the amfn-wasm.

However, it's also possible to extend an existing locale by using query parameters to specify your own files that contain user preferences, custom template groups, etc. Please refer to the AmFn Wasm Product Docs for more information on query parameters.