Quoting the caption from the paper: A spectrum of differing impacts and likelihoods from biothreats. Below each category of risk is the number of human fatalities. We loosely define global catastrophic risk as being 100 million fatalities, and existential risk as being the total extinction of humanity. Alternative definitions can be found in previous reports, as well as within this journal issue.
The paper Existential Risk and Cost-Effective Biosecurity makes a distinction between Global Catastrophic Risk and an Existential Risk in the context of biological threats: