Discipline-Specific Knowledge
In writing your SoP, there might be discipline-specific knowledge that you want to communicate for a variety of reasons. The purpose here could be to demonstrate your knowledge of the discipline or your ability to think critically. For example, you might discuss a problem in your field of study, offer a summary of relevant research which has informed your developing research interests, or identify a gap in the literature of field of study that motivates your research interests. This is not a required strategy, but it can be useful depending on how you want to contextualize your motivation, especially your research interests and academic and/or professional objectives. Often this strategy is a jumping off point for your motivation.
Notice in the examples below that the writers are pulling from information they have learned during their course of study.