Generally if the SCorp has income from an SSTB that exceeds 10%, then the entire business is considered an SSTB. here is a example from code 26 CFR 1.199A-5 (C) (1): and this is a link:

https://www.law.cornell.edu/cfr/text/26/1.199A-5

  (A) Example 1. Landscape LLC sells lawn care and landscaping equipment and also provides advice and counsel on landscape design for large office parks and residential buildings. The landscape design services include advice on the selection and placement of trees, shrubs, and flowers and are considered to be the performance of services in the field of consulting under paragraphs (b)(1)(vi) and (b)(2)(vii) of this section. Landscape LLC separately invoices for its landscape design services and does not sell the trees, shrubs, or flowers it recommends for use in the landscape design. Landscape LLC maintains one set of books and records and treats the equipment sales and design services as a single trade or business for purposes of sections 162 and 199A. Landscape LLC has gross receipts of $2 million. $250,000 of the gross receipts is attributable to the landscape design services, an SSTB. Because the gross receipts from the consulting services exceed 10 percent of Landscape LLC's total gross receipts, the entirety of Landscape LLC's trade or business is considered an SSTB.