(26)Regulation (EU) No 909/2014 requires the cooperation of authorities that have an interest in the operations of CSDs that offer services in relation to financial instruments constituted under the laws of more than one Member State. Nonetheless, the supervisory arrangements remain fragmented and can lead to differences in the allocation and nature of supervisory powers depending on the CSD concerned. Such fragmentation creates barriers to the cross-border provision of CSD services in the Union, perpetuates the remaining inefficiencies in the Union settlement market and has a negative impact on the stability of Union financial markets. While Regulation (EU) No 909/2014 provides for the possibility of setting up colleges, that option has rarely been used. In order to ensure the effective and efficient coordination of the supervision by competent authorities, the setting up of colleges should become mandatory under certain conditions. A college of supervisors should be established for CSDs the activities of which are considered to be of substantial importance for the functioning of the securities markets and the protection of investors in at least two host Member States. A college set up under this Regulation should not prevent or replace other forms of cooperation between competent authorities. ESMA should develop draft regulatory technical standards to specify the criteria on the basis of which it can be determined whether the activities are of substantial importance. Members of a college should have the possibility of requesting the adoption by the college of a non-binding opinion concerning issues identified during the review and evaluation of a CSD or during the review and evaluation of providers of banking-type ancillary services, or concerning issues that relate to the extension or outsourcing of activities and services provided by the CSD, or concerning any potential breach of the requirements of Regulation (EU) No 909/2014 arising from the provision of services in a host Member State. Non-binding opinions should be adopted by a simple majority vote.