$($a$)$ The criterion used when selecting the most appropriate RFx method is aligned with the organization's desired sourcing business model or sourcing continuum. This involves understanding where the organization stands on the continuum and choosing a method that aligns with the strategic approach to working with external suppliers or making investments.

$($b$)$ In the 'Shifting the continuum in practice' section, the article discusses how organizations consciously choose to move along the sourcing continuum, shifting from basic or approved provider models to more strategic ones such as the preferred, performance$-$based, or Vested business model. As organizations move towards more strategic relationships with suppliers, the dynamics of the relationship change, and contract structures shift from transactional and commodity$-$based thinking to more trusted, relational partnerships. This shift is driven by the desire to achieve productivity improvements and innovation, with suppliers taking on more significant risks, such as painshare penalties tied to performance against service level agreements under performance$-$based contracts. The article emphasizes aligning competitive bidding factors with each sourcing business model to determine the most appropriate RFx method based on the organization's strategic goals reword this as a studnet.