The variable length header is compulsory and must be at the beginning of the data stream. The header contains the following
| Type | Sub Type | Size | Contents | Content Meaning |
| Magic Number - required | 4 bytes | "xof " | ||
| Version Number - required | Major Number | 2 bytes | 03 | Major version 3 |
| Minor Number | 2 bytes | 02 | Minor version 2 | |
| Format Type - required | 4 bytes | "txt " | Text File | |
| "bin " | Binary File | |||
| "com " | Compressed File | |||
| Compression Type - required | 4 bytes | "lzw " | ||
| if format type is compressed | "zip " | |||
| etc... | ||||
| Float size - required | 4 bytes | 0064 | 64 bit floats | |
| 0032 | 32 bit floats |