SEO question ref: we are building a business/company listing directory in EdX Wix (over 1,000+ companies)

We are asking the SEO community to help us understand how we can best implement this for companies who list their business on our directory.
• What do you consider the primary keyword? (we want to give it a different name, but still try and be as specific as possible)
• What is the secondary keyword? (a more specific version of the primary keyword)
• What is the tertiary keyword? (a less specific version of the primary keyword)
Our goal in building this directory is to bring more clients/customers to our client’s websites. Listing with us should be a positive thing. This SEO community( https://marketingunfolded.com/b2b-seo-strategy/ ) will play an important role in helping companies understand which keyword we should use, as well as how to best describe their company so consumers will want to choose them.