That is already described in the document in section 3.3.1
In case there are no IPA Providers preselected the Provider Selection Service has to follow a Provider Selection Strategy as detailed below to determine those IPA Providers that are best suited to answer the request. The resulting list of IPA Providers candidates is asked in parallel and those that return the n-best results are selected (n ≥ 1).
That is already described in the document in section 3.3.1 In case there are no IPA Providers preselected the Provider Selection Service has to follow a Provider Selection Strategy as detailed below to determine those IPA Providers that are best suited to answer the request. The resulting list of IPA Providers candidates is asked in parallel and those that return the n-best results are selected (n ≥ 1).