The relaxation dispersion model for determining the $R_{2,eff}$ or $R_{1\rho}$ values used as the base data for all other dispersion models. This if for all SQ CPMG-type data, MMQ CPMG-type data, and R1rho-type data. This model is labelled as R2eff in relax.
Parameters
The R2eff model has the parameter $R_{2,eff}$ or $R_{1\rho}$ depending on the data type. It may also have the $I_0$ initial peak intensity parameter if exponential curves have been collected.
Reference
These is no reference for the R2eff model.
Links
The implementation of the R2eff model in relax can be seen in the: