Reddit mentions of Imperial Japanese Navy Antisubmarine Escorts 1941-45 (New Vanguard Book 248)

They're technically neither because they were incapable of nor intended to fully perform as destroyer escorts, and lacked the displacement for frigates. Their role was a longer range, oceangoing cross between coastal defense ships and subchasers, neither of which had oceangoing capability, but their designs were essentially slightly enlarged versions of the subchasers. All of them were basically auxiliary escorts. While the first four that comprised the Shimushu-class were built in the late 1930's to deal with any minor threats posed by Soviet naval forces, the subsequent ones built during the war were intended as a stopgap to deal with the increasing toll USN submarines were inflicting on their convoys. Destroyers losses had taken their toll on the IJN so they couldn't be relegated to convoy escort duties, and subchasers could only operate in coastal areas. Thus the IJN designed Types A-D, the oceangoing subchasers, essentially a stopgap destroyer escort lacking torpedoes. Both navies intended for them to be budget destroyers, but only the USN could afford a true multi-role vessel, whereas the IJN had to skip the torpedoes. This was not seen as an issue, as the IJN had realized that the greatest threats to their ships came from submarines and aircraft.

They are not destroyer escorts as they lack the true multi-role capability, especially torpedoes. Calling them frigates is too broad, as there were frigates designed specifically for ASW, or AA, etc., in addition to lacking the displacement, although it would be a more apt term. They were labeled PF under some USNTMJ reports, but the USN also didn't know much about them at the time, only that they were larger subchasers. Kaibokan/Kaiboukan is too broad of a category, as it includes light cruisers as well. In fact, during the war, the entire kaibokan classification was removed from IJN terminology and replaced with simply "escort," although the term continued to be used unofficially. The Imperial crest was subsequently removed from the bow of all kaibokan ships. The closest classification in the USN in terms of role would be the PCE, patrol craft escort, an oceangoing . Ironically, PCEs were larger than their PC and SC (subchaser) counterparts, and could operate in the open ocean, but since they were under 1000 tons, they received their own classification. Keep in mind though, the terms frigate/destroyer escort/corvette/sloop/gunboat/coastal defense ship have all been used interchangeably for the same ships. By the WWII definition, a frigate could be any oceangoing warship smaller than a destroyer that is capable of surface combat, ASW, and AA. The destroyer escort was likely the most successful "frigate" of WWII.

I voted for Something Else, perhaps Auxiliary Escort (AE), Corvette (DDC or PC/PCE), or simply Escort (EE), but I'm okay with Patrol Frigate (PF) if there's no consensus.

Compiled ONI reports from IJN records between 1941-1944 categorized them with the ASW subchasers and special subchasers: http://www.ibiblio.org/hyperwar/USN/ref/KYE/CINCPAC-142-45/

I also have a friend who is working with Mark Stille on his upcoming book on IJN ASW escorts: https://www.amazon.com/Imperial-Japanese.../dp/B06X9R7QDZ/

USNTMJ photos of Kaibokan ships: http://www.fischer-tropsch.org/primary_documents/gvt_reports/USNAVY/USNTMJ%20Reports/USNTMJ-200H-0602-0659%20Report%20S-03.pdf

USNMTJ reports of ASW and kaibokan performance and capabilities: http://www.fischer-tropsch.org/primary_documents/gvt_reports/USNAVY/USNTMJ%20Reports/USNTMJ-200I-0244-0309%20Report%20S-24.pdf