What is model column in MigrationHistory table?

On MSSQL use

SELECT *, CONVERT(xml, DECOMPRESS(Model)) FROM [dbo].[__MigrationHistory]

Internally it uses this function to get value for Model field:

    public virtual byte[] Compress(XDocument model)
    {
        DebugCheck.NotNull(model);

        using (var outStream = new MemoryStream())
        {
            using (var gzipStream = new GZipStream(outStream, CompressionMode.Compress))
            {
                model.Save(gzipStream);
            }

            return outStream.ToArray();
        }
    }

So yes, it seems that the field contains whole model in a compressed form.