Business Analyst and System Analyst ?

What are the roles of Business Analyst and Systems Analyst in the Analysis phase? Let us say that i am intending to develop an MIS ot ERP for the Textile or Medical industry. Then what these 2 stakeholders be doing in the Analysis phase? Also can a Business Analyst also act as a System Analyst or vice versa (is it advisable or not)? Also typically what UML diagams does a Business Analyst and a System Analyst use to model the system?

Business Analyst is actualy business person who understands well business aims and objectives. System analyst is a technical person which completely knows about old system and can analyze from requirement perspective what new features should be added. By considering your example, Business analyst will work on analysis and refinement of  business requirements where system analyst will analyze new system requirements by keeping view the old system's functionality. Both work together to analyze and refine requirements that what are actual requirements of new system to meet business goals and objectives.  Business analyst can be system analyst too if he is technical person. In requirement engineering phase, instead of diagrams SRS document is used. Once requirements are finalized and verified then UML is used to further eleborate system which is design phase