It is an IME that facilitates inputting mathematical expressions.

Keys are divided into different key-groups.  The Criteri-R key-group targets the Criteri-R app.  The User key-group allows users to  define their desired key texts like macros.  Alphabets and General punctuations and symbols are added for normal inputs.

Besides the common math input keys, on the keyboard there are a number of mutable keys whose contents can be changed to map to any keys in the selected key-group.  Users can define as many keys as they need in the User key-group.  In this way, Math Keyboard can be used to serve Criteri-R as well as other apps, e.g. Excel, WolframAlpha, etc.

Superscript and subscript counterparts, if available for a character, can be attained through the Shift key.

Through in-app purchase, more characters become available: arithmetical, Greek, arrows, brackets, mathematical, accounting, calculus, logical, set theory and multi-line characters.  They will all be displayed on your device before you agree the purchase.

Remember to enable and select Math Keyboard in System Settings after installing.