Each pass has a front and a back of the card. The basic anatomy of a pass is fixed and can only be changed to a certain degree. That is why it's great to be aware of the format's possibilities before creating a template.
The front side is all about colours, images and brief, most frequently used information. With templates like store card or event ticket you can use banner images, generic template supports a passport style image and a boarding pass just features a thin strip image. You can use plain text and emojis in the text fields.
The back is much more content focused. You cannot change the background colour or include images, but you can add an unlimited number of text fields. The text fields do not have a character limit either and you can also use plain text, emoji and hyperlinks, so you can use the back to include more detailed information, contact details, links to documents, websites, map navigation etc.