As of update 1.8, the .rm files are in a new format.

It lookes like instead of each file being indexed by the page number, the pages are now assigned a random UUID.

[root@remarkable 6b96d778-b6f5-458e-9b7d-649689706f37] ls
012f4a15-8cdb-4d44-aa02-05fd911873e6-metadata.json
012f4a15-8cdb-4d44-aa02-05fd911873e6.rm
0fe5e46a-51c1-4255-a062-c1333dd7eac9-metadata.json
0fe5e46a-51c1-4255-a062-c1333dd7eac9.rm
52d23c5a-9e79-46cd-9aca-560cf572cddf-metadata.json
52d23c5a-9e79-46cd-9aca-560cf572cddf.rm
564b27fd-8ad0-44f1-a041-6a5e022a131d-metadata.json
564b27fd-8ad0-44f1-a041-6a5e022a131d.rm
57791cbe-8297-41c8-8a0b-98a4eb4bf271-metadata.json
57791cbe-8297-41c8-8a0b-98a4eb4bf271.rm
dec625e0-8c4a-4cba-8143-bf8c8b9b0d86-metadata.json
dec625e0-8c4a-4cba-8143-bf8c8b9b0d86.rm

Additionally, the accompanying .json files contain layer information:

[root@remarkable 6b96d778-b6f5-458e-9b7d-649689706f37] cat 012f4a15-8cdb-4d44-aa02-05fd911873e6-metadata.json 
{
    "layers": [
        {
            "name": "Layer 1"
        }
    ]
}

Page order can be inferred from the .content file

[root@remarkable 6b96d778-b6f5-458e-9b7d-649689706f37] cat ../6b96d778-b6f5-458e-9b7d-649689706f37.content  
{
    "extraMetadata": {
        "LastBrushColor": "Black",
        "LastBrushThicknessScale": "2",
        "LastColor": "Black",
        "LastEraserThicknessScale": "2",
        "LastEraserTool": "Eraser",
        "LastPen": "Ballpointv2",
        "LastPenColor": "Black",
        "LastPenThicknessScale": "2",
        "LastPencil": "SharpPencilv2",
        "LastPencilColor": "Black",
        "LastPencilThicknessScale": "2",
        "LastTool": "Ballpointv2",
        "ThicknessScale": "2"
    },
    "fileType": "pdf",
    "fontName": "",
    "lastOpenedPage": 0,
    "legacyEpub": false,
    "lineHeight": -1,
    "margins": 100,
    "orientation": "portrait",
    "pageCount": 11,
    "pages": [
        "00dd2a52-3057-4fac-8438-d7177a631df0",
        "dec625e0-8c4a-4cba-8143-bf8c8b9b0d86",
        "52d23c5a-9e79-46cd-9aca-560cf572cddf",
        "012f4a15-8cdb-4d44-aa02-05fd911873e6",
        "564b27fd-8ad0-44f1-a041-6a5e022a131d",
        "57791cbe-8297-41c8-8a0b-98a4eb4bf271",
        "0fe5e46a-51c1-4255-a062-c1333dd7eac9",
        "ed1694c3-b67a-450d-bb45-e4691c5cc75f",
        "351c255b-3350-4771-b6cc-b02625c1a6d5",
        "41259360-ba5d-45d2-9fe6-57434f79950b",
        "9399bb75-310a-4d31-8961-5f26b6f85500"
    ],
    "textScale": 1,
    "transform": {
        "m11": 1,
        "m12": 0,
        "m13": 0,
        "m21": 0,
        "m22": 1,
        "m23": 0,
        "m31": 0,
        "m32": 0,
        "m33": 1
    }
}

Only pages that have been annotated have .rm files.