Cap for the nib is to protect your nib from damage and dust. It also protects your fingers while reaching the pen especially when you leave the nib on your pen holder.
To select the cap size you need, check the chart on this page of which group your nib belongs to.
Cap for Nib
Cap Size Suitable Nibs No.1 Hunt 99 / Hunt 103 / Hunt 100 / Hunt 1068 / Hunt 22 / Hunt 99 / Hunt School 56
Hunt 101 / Gillot 170 / Gillot 290 / Gillot 303 / Gillot 291 / Leonardt General
Leonardt 6H / Leonardt 33 / Italic 0286 / Leonardt 41 Crown / Leonardt EF / Leonardt 30 / Blanzy 2552 and similar in size / Brause 46 Cito
No.2 Leonardt IIIEF England / Leonardt 256 England / Nikko G
Zebra G (titanium) / Brause 76 / Tachikawa G / Leonardt 30 / Blanzy 2552 / Brause 46 Cito
and similar in size
No3 Leonardt 40 Blue Pumpkin / Hunt Globe 513 EF
and similar in size