Hey Skylimit thats too big a question!
Your gonna need to be more specific.
If you have a question about 5th line ask that!
Is it for your kite?
If so what is it Bow, SLE or C.

Hey skylimit 'JINX' I have the 11 and 15 fuels both 5 line.

If thats what you have Its a C kite not SLE. (supported leading edge)

Or a C with some sort of bridle at the front to maintain the flatter shape of the kite

Im guessing you have 4 line. When you drop your kite you have to do all sorts of fancy over your head with the bar then swim towards your kite stuff to get your kite on its back. Once there its pretty easy to work it towards the edge and relaunch.

5 th line you let go of everything and the only attachment you have to the kite is the one line to the leading edge. This puts the kite on its back on the water ready for the easy bit of the 4 line relaunch.
I found I learn stuff quicker because Im not bothered if the kite hits the water.

Smarter people than me also use the 5th line to trim the flying angle of the kite, some tension in the 5th line at the centre of the leading edge flattens the C shape presenting more material to the wind.

Get 5th line, Its money well spent.

Good luck, Dan