Courses that are transferrable from other colleges and universities, but don't have a UW equivalent will be posted as "X" credit. For example, an introductory political science course might be posted as POL S 1XX if the UW doesn't teach a course on exactly the same topic. An upper-division home economics course might be posted as UW 3XX because UW doesn't have a home economics department. Interdisciplinary courses are usually posted as UW 1XX.
X-credit courses can be counted toward requirements. In some departments this is done automatically. Since UW counts all political science courses as I&S, for example, a POL S 1XX course will automatically count toward I&S. Other departments, for example psychology and philosophy, have courses that count toward different general education requirements depending on the topic of the course. If you are transferring a course in one of these departments from a Washington community college, it should be assigned the same designation that appears in the equivalency tables. If you are transferring such courses from another college, it will have to be evaluated by an adviser.
A grade of "X" is recorded if no grades is submitted by the instructor. Usually, X grades just mean that the instructor is late submitting grades for the entire class. Occasionally, an instructor will submit an X grade when s/he wishes to postpone submitting the grade for some reason. If you receive an X grade, you should check with the instructor.