This is the video codec used (HEVC). It allows for high-quality video at a smaller file size compared to older formats.
For archival purposes, always check if the subtitles are properly synced and OCR’d without errors. Fan-made subtitles from opensubtitles.org sometimes lack proper timing for WEBRips. Parthenope.2024.1080p.WEBRip.5.1.x265.ESub-Katw...