Repetitive fields go first
It is widely recommended that you prioritize the association of the first tag positions with the fields that are supposed to be recurrently filled.
As you will see in the Encoding section, the positions from 1 to 15 will only take one byte (instead of two or more) for pointing out the position in the binary object.
In conclusion, if you have a message with more than 16 fields, and some of them are optional, don’t forget to place the optional ones at the end of the message.