Confined spaces are large enough for an employee to enter fully and perform assigned work, are not designed for continuous occupancy by the employee, or have a limited or restricted means of entry or exit. Permit-required confined spaces contain or have the potential to contain a hazardous atmosphere, contain a material with the potential to engulf someone who enters the space, have an internal configuration that might cause an entrant to be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross-section, or contains any other recognized serious safety or health hazards.
Want to learn more? Check out the OSHA guideline here: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.146