Complex sentence: Lawson refers to these companies as "S3," described as "staffing, sourcing and serving" companies, including healthcare, financial services, public services, retail and industry markets.
Split into simpler sentences: Lawson refers to these companies as "S3," which stands for companies that "staff, source and serve" as part of their business. Lawson's target S3 "vertical" industries include healthcare, financial services, public services, retail and industry markets.