There are various types of small businesses that have the potential to generate $1 million in annual revenue. While it is important to note that success depends on several factors such as location, market demand, competition, and the business owner's skills, here are a few examples of industries where small businesses have the potential to reach this revenue milestone:

1. E-commerce and Online Retail: With the rise of online shopping, small businesses that operate in the e-commerce sector have the potential to generate significant revenue. For example, a niche online retailer specializing in a specific product category, such as luxury pet accessories or sustainable clothing, can attract a loyal customer base and achieve substantial sales. One successful example is Chewy.com, an online pet supply retailer that reported $3.5 billion in net sales in 2019.

2. Health and Wellness: The health and wellness industry has experienced substantial growth in recent years. Small businesses in this sector, such as fitness studios, wellness retreats, or specialized health food stores, can generate significant revenue. For instance, SoulCycle, a chain of indoor cycling studios, has achieved substantial success with studios across the United States and reported $112 million in revenue in 2018.

3. Technology and Software: Small businesses that develop and sell innovative software solutions or provide technology services can achieve considerable revenue. For example, a company that develops mobile applications or provides cloud computing services can generate substantial income. One notable example is Slack, a team collaboration platform, which reported $630 million in revenue in 2019.

4. Food and Beverage: The food and beverage industry offers numerous opportunities for small businesses to generate significant revenue. For instance, a successful restaurant or a chain of fast-casual dining establishments can achieve annual sales in the millions. Sweetgreen, a fast-casual salad chain, reported over $300 million in revenue in 2019.

5. Professional Services: Small businesses that offer specialized professional services, such as consulting, marketing, or legal services, have the potential to reach the $1 million revenue mark. For instance, a marketing agency that caters to a niche market or a law firm specializing in a specific area can attract high-paying clients and achieve substantial revenue.

It's important to note that these examples are not exhaustive, and success in any industry requires careful planning, execution, and adaptability to market conditions. Additionally, factors such as location, target market, competition, and the uniqueness of the business idea play a significant role in achieving substantial revenue.

References:
- "Chewy Announces Fourth Quarter and Fiscal Year 2019 Financial Results" - Chewy Investor Relations: https://investor.chewy.com/news-releases/news-release-details/chewy-announces-fourth-quarter-and-fiscal-year-2019-financial
- "SoulCycle Reports Fourth Quarter and Full Year 2018 Results" - SoulCycle Investor Relations: https://investor.equinox.com/news-releases/news-release-details/soulcycle-reports-fourth-quarter-and-full-year-2018-results
- "Slack Announces Fourth Quarter and Fiscal Year 2019 Results" - Slack Investor Relations: https://investors.slackhq.com/news-releases/news-release-details/slack-announces-fourth-quarter-and-fiscal-year-2019-results
- "Sweetgreen 2019 Impact Report" - Sweetgreen: https://impact.sweetgreen.com/2019-impact-report/