The Reportable REST API only uses a subset of the XPath 2.0, and I can't see any mention of the "contains" function in the whitepaper (in fact, it says "3.5.1 Value Comparisons" is not suppported), so I believe the contains() function is not supported. I will get a "CRRED0121E: Invalid filter condition" error if I attempt to use "contains" as a filter.

Field selection and filtering

Section 4.3 of the whitepaper document describes the field selection capability which lets you define which fields are returned in the data set. It uses a subset of XPath 2.0. This mechanism also enabled filtering capability (i.e. selecting certain fields where the value matches certain criteria). Please refer to section 4.3 for more detailed information about field selection and filtering.