I am reading through BIP32's documentation and its implementation in Java on GitHub in the file supernode/api/src/main/java/com/bitsofproof/supernode/api/ExtendedKey.java.

Luckily, I was able to follow the code, but there are areas I am not sure about.

I have 2 questions:

1. I know it is in BIP32.json file, but why are we doing if ((sequence & 0x8000000)==0)? It is a condition for master public key to child public key or master private key to child private key... but what sequence does for? if ( (sequence & 0x80000000) == 0 )

2. My second one is why we add 4 bytes. Is there any detail document source I can find?

           extended = new byte[pub.length + 4];
           System.arraycopy (pub, 0, extended, 0, pub.length);
           extended[pub.length] = (byte) ((sequence >>> 24) & 0xff);
           extended[pub.length + 1] = (byte) ((sequence >>> 16) & 0xff);
           extended[pub.length + 2] = (byte) ((sequence >>> 8) & 0xff);
           extended[pub.length + 3] = (byte) (sequence & 0xff);
       }
       else
       {
           byte[] priv = master.getPrivate ();
           extended = new byte[priv.length + 5];
           System.arraycopy (priv, 0, extended, 1, priv.length);
           extended[priv.length + 1] = (byte) ((sequence >>> 24) & 0xff);
           extended[priv.length + 2] = (byte) ((sequence >>> 16) & 0xff);
           extended[priv.length + 3] = (byte) ((sequence >>> 8) & 0xff);
           extended[priv.length + 4] = (byte) (sequence & 0xff);
       }
   

Many thanks in advance.