Recommendation ITU T G.994.1 provides a flexible mechanism for digital subscriber line (DSL) transceivers to exchange capabilities and to select a common mode of operation. It includes parameters relating to service and application requirements as well as parameters pertinent to various DSL transceivers. This Recommendation is currently an integral part of the start-up procedure for Recommendations ITU T G.991.2, ITU-T G.992.1, ITU-T G.992.2, ITU-T G.992.3, ITU-T G.992.4, ITU-T G.992.5, ITU-T G.993.1 and ITU-T G.993.2. It is anticipated that future DSL Recommendations will also be able to make use of this Recommendation. Provisions are also included for exchanging non-standard information. The 2018 edition of this Recommendation integrates ITU-T G.994.1 (2017) and all its amendments and corrigendum. In addition, the 2018 edition includes the following new technical material: -codepoints for support of US0 of G.993.2 Annex N PSD masks -codepoints for support of US0 together with profile 35b of G.993.2 Annex A PSD masks -codepoints for support of G.9701 extended probesequence length