What does the prefix of the raw transaction coding of an exported Electrum transaction file (.txn) mean?

I have exported a testnet transaction to a .txn file:

{
"hex": "45505446ff0002000000000101c6be24d86ab1ae5b54e7e7905b ...",
"complete": false,
"final": false
}

However, as explained in this article, an unprocessed transaction string begins with the version number and:

The version number has a length of four bytes and is expressed as a hexadecimal value in little endian format.

There are two types of versions. Version 01 indicates that there is no relative time blocking. Version 02 indicates that there may be a relative time lock.

So, this hex is not a qualified unprocessed transaction code, due to the prefix

45505446ff00

The version number comes after this prefix:

02000000

So I was wondering, what is the function of this prefix that Electrum adds to the unprocessed transaction chain? And is it always the same length? Because I need to join it automatically to get a valid raw transaction string.