Is an Intelligent mail barcode formed using Digits (36 numberic) or Alpha (75 alpha) characters?

Intelligent Mail (aka OneCode), it is formed solely with digits 0-9. The data length must be exactly 20, 25, 29, or 31 digits.

No letters, spaces or punctuation are allowed. Only numeric digits 0-9.

If you would like to experiment with how edoc will react to OneCode inputs, here is a test template: