In some cases, all the partners of a limited partnership may be investors only and not managers. The partnership may hire an outside general manager (employee) to make the day-to-day decisions, and therefore, acts as the manager. In this case, the general manager is an employee of the business, but since they are not a partner (aka owner), their income is just like that of an employee in any business.
When you have an investor in your partnership, who works less than 500 hours in a given tax year for the partnership, and they do not participate in its management, they are considered limited in their liability and their income is usually considered passive income subjecting the income to only federal and state income taxes based on their marginal tax rate.
A partner in a limited partnership that is a decision-maker (general partner) is considered an employee of the business by the IRS and is treated differently than limited partners. Income for general partners is considered earned income and is subject to additional taxes but also to additional potential tax deductions. Moreover, as a general partner, you are exposed to additional liabilities.