The katl-commons (SIL Engine™) library offers a suite of features that you can use to develop your own plugins based on Simple Issue Language™ (SIL™).
You can create free plugins based on SIL Engine™ and this documentation but you cannot create commercial (paid) plugins based on SIL™. If you wish to do so, contact us at firstname.lastname@example.org, and we will discuss the joined approach with you.
Many people don't realize, but SIL Engine ™ is free software that you can use it to create your own non-commercial plugins (we reserve all rights for the commercial plugins).
We may change signatures of routines, alter the descriptor objects and so on.
Script interpretation remains the same. You should base your code on the assumption that the interpreted script will yield the same results.