Tags put order data on your documents. Use them in the Comment field. Document numbering has more tags: see Content » Document numbering.
Tags you can use in the Comment
{{order_number}}
The order number Shopify creates automatically. For example: 1234
{{order_id}}
The order ID Shopify creates: the order number with your prefix and suffix. Format: PREFIX + Order Number + SUFFIX. For example: BB1234HELLO
Set it in Shopify Admin » Settings » General » Order ID:
Example
Your Shopify order number is 565.
You enter “Shopify order: {{order_number}}” in the Comment field and save.
The comment on the document will be “Shopify order: 565”.