APN is an OctectString data type and it maximum length is 100 as per section 9.1 of 23.003 specification.
In document we usually seen APN in following format.
Now if above format in label format will looks like
label1. label2. label3. label4
As per section 9.1 of 23.003 spec
"Each label is coded as a one octet length field followed by that number of octets coded as 8 bit ASCII characters".
It means if "province1" is the label1 then it will coded as following
length = 9 (one octet) and followed by 9 Octets which holds 9 characters start from "p" and end with "1".
Hope this information will help you.