The new SAT II exam includes a writing section, and one on critical reading. These two sections are part of the general SAT exam and ALL students taking the SAT II should be familiar with them.
As with the original SAT, there are several optional subject matter tests. These are separate tests for which you must register in advance, although they may be administered with the SAT II. (Makes for a long day!!) It is recommended that students attempt only one or two of these subject exams at one sitting. If a student wishes to take more, they should be scheduled for another testing day.
"Optional" generally means the admitting institution's option. Some of the more competetive schools require some of these SAT subject tests. Be sure to investigate the requirements of the schools to which you are applying in plenty of time to register and prepare for any SAT subject exams you may need to take.