What characterizes the outsourcing model of transaction relationships?

The outsourcing model of transaction relationships is characterized by utilizing the firm's internal staff on a transaction basis. This model allows organizations to tap into external talent or resources while still leveraging their own internal capabilities, essentially blending the use of internal and external resources based on project demands or specific needs.

In this context, employing internal staff on a transaction basis means that while the firm maintains some control and involvement, the actual execution of tasks can be adjusted based on the demands of the situation. This flexibility can enhance efficiency and responsiveness to market changes or project requirements.

The other options reflect different dynamics of outsourcing that do not capture the essence of the transaction-based utilization of the firm's internal resources. For instance, co-managing a project implies a deeper collaborative relationship than standard transaction relationships might entail, and contracting for well-defined IT services focuses on the service aspect rather than staffing. Similarly, relying solely on a vendor’s systems shifts the focus away from the firm's internal capabilities and does not embody the transaction-based use of internal staff.