Effectively, this is the file size required to suit and sharpen the chipper tooth.
Plenty of info on youtube or whatever and various file holders are available. These help to some degree but practice, as ever takes time to perfect. I like to hold the chain bar in a vice, slightly uphill, and keep the chain tight as I file away.
If you can find a firm thats open, they’ll machine sharpen the chain for a tenner, and sell you a spare for twice that. Always a good idea to keep one in!
John