I contacted both Eureka City Clerk Pam Powell and Humboldt County Clerk/Recorder/ Registrar of Voters Kelly Sanders regarding this post. Ms. Powell deferred to Ms. Sanders.
This is Ms. Sanders'response:
"If the person is not living in a residence or a shelter, they can use the street coordinates closest in proximity to where they are staying. It can be a street corner (Example: 3rd & A Streets, Eureka). They should not use a business address (non-shelter) as a residential address. If a business is allowing them to use their address to receive mail, they can put that address down as their mailing address."
I requested further clarification from Ms. Sanders:
"They need to list the location where they live.
We need to know where you live so we know what precinct you live in and what districts you are eligible to vote on. "
"A PO Box or a business address can be used for your mailing address."
"(b) A person does not gain a domicile in any precinct into which he or she comes for temporary purposes merely, without the intention of making that precinct his or her home."