Under the Joint Comprehensive Plan of Action (JCPOA), Iran’s procurement of materials that can be used in the manufacture of gas centrifuges is restricted. Gas centrifuges are used for the enrichment of uranium, a common process in the production of nuclear fuel materials.
Materials and equipment which have a nuclear application must be acquired by Iran through the official "Procurement Channel", allowing the United Nations Security Council to maintain oversight and restrictions on the imports of such materials to the country. Any such procurements made outside of the official Procurement Channel would be a serious violation of the terms of the JCPOA.
The United States recently designated several Chinese entities for supplying goods to Iran. The materials are alleged to have either been sourced directly by the Iranian Enrichment Technology Company, TESA, or procured through Iranian sub-contractors, in one case using an international front company in Belgium.
Company registration data suggests these alleged transfers would have happened after the JCPOA was adopted. This would mark a serious violation of the agreement.
You can read the full case study here: US Designates New Iran Centrifuge Material Supply Networks