Short Answer
Outside the context of an electric utility’s regulatory compliance under an RPS, it is not clear that you are buying anything. Formally, you are paying for a certificate that records the generation of one MWh of electricity from a qualified renewable resource. However, you are not purchasing, taking possession of, or using “electricity” simply by purchasing a REC or GO. Further, you are not buying zero-emissions power, emission reductions, or avoided emissions.
Long explanation
A REC or GO is not a purchase of green power (and you are not buying “electrons” as commonly suggested[1]). RECs and GOs are a form of financial contract that takes place independently of electricity distribution and consumption physics; there is no system that can track (see Does buying a REC or GO mean I am using renewable energy?) the origin of electricity on the consumption side of a pooled electrical transmission and distribution grid.
Instead, the common marketing language associated with RECs and GOs is that they “represent” environmental, green, or renewable “attributes” or “benefits” associated with renewable energy generation. See What are the “environmental benefits” or “attributes” associated with RECs and GOs? for what is an “attribute”. In economics terminology, you are not clearly buying a good or a service. Instead, economically speaking, you are making a financial contribution to a company producing electricity with renewable resources, which then begs the question of whether that donation has a beneficial impact (see Does buying a REC or GO mean I am using renewable energy?.)
Green-e® claims in their Code of Conduct that “Renewable Energy Certificates (REC) do not contain electricity. REC represents the environmental benefits of 1 megawatt hour (MWh) of renewable energy that can be paired with electricity”(Green-e® Code of Conduct, p27, 11 December 2020). Yet, in its marketing materials, Green-e® also states that “RECs are used to demonstrate use of renewable electricity in the U.S.” (CRS, 7 March 2016). This type of confusing, and frequently misleading, language is ubiquitous across voluntary green power marketing materials.
[1] Gillenwater, M. (2013). Is the way you think about emissions from purchased electricity wrong? Greenhouse Gas Management Institute, February 2013. https://ghginstitute.org/2013/02/26/is-the-way-you-think-about-emissions-from-purchased-electricity-wrong/