From A360:
Applications must be able to specify that a text field accepts a certain class of input (e.g. e-mail, phone number, URL, plain text). A virtual keyboard, if present, should adjust its presented set of characters to the input class. Input method constraints are hints: an implementation is not required to restrict the set of characters that can be entered. (API requirement on FX, implementation requirement on convergence).

Update:
This requirement must also include constraints of scripts (e.g., katakana) so that only input of a single script are accepted.

Controls team update:
The Input Method Constraints support is a intended to be used by developers as a means to provide a hint to keyboards (particularly virtualised keyboards) such that they may customise what is displayed to the user. For example, if a TextField is intended to be used for accepting a phone number, an input method constraint may be set on it to state this intention. When a virtualised keyboard is shown for this field, it may change from alphanumeric to being simply a number pad.

The UI Controls team met on February 2, and discussed this and what we believe the requirements are. The discussion centered around the following points:

 * There is concern about how this may relate to the formatted text requirement (RT-14000), and this needs to be explored further.
 * Very basic on the desktop.

Based on the UI controls team discussion, the 'planning poker' value for this feature was deemed to be 3.