In 2.1 Purpose, it references Clinical Quality Language (CQL) R1.5.2, but there are a few places else where CQL version is mentioned, it is referencing older CQL version. 

2.1 Purpose

"Although the specification is based on the 1.4 version of CQL, backwards-compatible future versions of CQL can be used as well. In addition, if necessary, the 1.2 and 1.3 versions of CQL can be used without loss of functionality for this Implementation Guide." 

2.6.1 Clinical Quality Language R1.4

Clinical Quality Language R1.4 (CQL) is an HL7 standard for trial use (STU). It is part of the effort to harmonize standards between quality measures (QMs) and clinical decision support (CDS). CQL provides the ability to express logic that is human readable yet structured enough for processing a query electronically.

3.1.1 Related Documents

Clinical Quality Language R1.4 can be used in conjunction with the FHIR Measure resource to construct CQL-based quality measures. CQL is a domain specific language used in the Clinical Quality Measurement and Clinical Decision Support domains. Measures written in CQL leverage the expressivity and computability of CQL to define the population criteria used in the QM.

4.12.5 MIME Type version

The version of CQL/ELM used for content in a library should be specified using the version parameter of the text/cql and application/elm+xml, application/elm+json media types.

• text/cql; version=1.4
• application/elm+xml; version=1.4
• application/elm+json; version=1.4