Is the HBM Voltage AC or DC?

This standard covers the requirements necessary to establish, implement, and maintain an electrostatic discharge (ESD) control program for activities that manufacture, process, assemble, install, transport, package, label, service, test, inspect, or otherwise handle electrical or electronic parts, assemblies, and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts human body model (HBM) and 200 volts charged device model (CDM). The CDM voltage level used in this document is based on managing process essential insulators to mitigate field-induced voltages on devices that could lead to damage.

Based on ESD S20.20, what type of voltage is HBM pertaining to? AC or DC Voltage?

It would be more of a DC voltage. The Human Body waveform used to determine device sensitivity is a single pulse, so it is neither AC or DC but just a pulse or transiate waveform.