In this contribution, different approaches based on the X-parametersTM to model the behavior of mismatched nonlinear transfer systems are examined. The X-parameters based on the PHD1-principle introduced by Verspecht and Root (2006) as an extension of the well-known S-parameters describe nonlinear microwave 2-port-networks under large signal conditions. Using load-pull measurement techniques they can be used for arbitrary load situations. Beside this load-pull approach, in the work of Cai et al. (2015), it is stated that it is sufficient to use one optimized X-parameter set for each value of the load reflection coefficient without introducing a large error. In another contribution of Cai and Yu (2015), this approach is extended to cover the whole smith chart with one optimized X-parameter set instead. In this work, these different approaches are compared and brought into question.