This is the video codec. H.265 (HEVC) is the successor to H.264 (AVC). For a film like Spectre, x265 offers:
You have a large library but limited storage. A 4 GB file vs a 30 GB file means you can store 7-8 times more movies. However, note that x265 10-bit requires transcoding on older streaming devices. A modern Nvidia Shield, Apple TV 4K, or Fire Stick 4K will play this natively. Spectre.2015.1080p.10bit.BluRay.8CH.x265.HEVC-PSA
The most significant technical term in the filename is x265 (and its container standard, HEVC/H.265). This is the video codec
| Release Name | Size | Codec | Audio | Color Depth | Verdict | | :--- | :--- | :--- | :--- | :--- | :--- | | Spectre.2015.1080p.BluRay.REMUX | 30 GB | x264 | 7.1 TrueHD | 8-bit | Gold standard, but massive. | | Spectre.2015.1080p.BluRay.x264-SPARKS | 8.7 GB | x264 | 5.1 DTS | 8-bit | Great, but obsolete vs x265. | | Spectre.2015.1080p.10bit.BluRay.8CH.x265-PSA | 4 GB | x265 | 7.1 AC3 | 10-bit | Best size/quality ratio. | | Spectre.2015.2160p.UHD.BluRay.x265-Someone | 18 GB | x265 | 7.1 Atmos | 10-bit HDR | Better if you have HDR TV. | A 4 GB file vs a 30 GB
Note on HDR vs SDR: This PSA release is SDR (Standard Dynamic Range) because the source Blu-ray is SDR. If you want HDR, you need the 4K Blu-ray. However, the PSA 10-bit SDR still looks spectacular on HDR displays.