It limits the size of an individual string's presentation form. It gets used to temporarily store a TXT string as a byte slice in [packTXTString](https://github.com/andrewtj/dns/blob/atj-encoding/msg.go#L480-L481). I haven't actually benchmarked to see if this makes any speed difference so I'm happy to rename it (txtStringTmp is more appropriate), remove it or write a benchmark and go from there.