After selecting source files, you design how the recovery files will become.
How much redundancy you need
The most important setting in creating recovery files is "Redundancy". As you set more redundancy, it will be possible to repair, but more time and space are required. The required quantity depends on your circumstance; like error rate of the saving drive or reliability of the network. If you create as an insurance against rare incident, 5% or 10% may be enough. If you get error often, you may require more, such as 30% or 50%. If you want to protect seriously, backup source files and create recovery files.
A whole file can become unreadable by a hardware failure. Each whole file can be lost by a human mistake like careless deletion or forgeting to transport. Then, in actual repair, the problem may be whether you have enough redundancy to reconstruct the whole file. The measure is "Number of files that can be fully reconstructed if missing" on the bottom of Create window. The "Min - Max" means that, how many files you can repair at least min and under than max. The min should be 1 or more for safety.
How is the efficiency
There is a measure of recovering capability, that is "Efficiency" on the bottom of Create window. Unless you have a trouble to keep saving space, you don't need to take care usually. When there is a big difference between settings, you would better to select a efficient way, but you can ignore small difference like several percent. Because the possibility of repair is largely varied by distribution of damage, it is worthless to aim for the best efficiency.
If you post the recovery files on UseNet
It is important to set the unit of blocks for UseNet on Option window. Though it depends on the server, mostly it is "For yEnc: 3,000 lines" or "For yEnc: 5,000 lines". No need to change other settings. If you want smaller recovery files, you may limit repetition of packets by selecting "Upto 4 times". Then, you should select "Powers of 2 sizing scheme" at "Sizing scheme" on Create window.