Thinking about the recently imposed trade tariffs by the Trump administration and counter-tariffs from Canada, I noticed they're all on physical tangible goods that pass through customs: metals, lumber, certain consumer products and so on.
But often a country or an economy has significant exports or imports in intangible goods and services that don't exactly "pass through" customs, with "information" (as opposed to matter) being the primary example, as it can take the form of software, databases, literature and so on. (Though at least with literature in the printed-form you could tariff the import/export of physical books or tax domestic publishers who print foreign volumes.) - so if tariffs were imposed on another country that relied heavily on software or information exports in general, how would it work?
For one thing, software is very difficult to value: software often has a stated cash value of $0.00 (especially open-source software and shareware) - often those companies make their money from selling licences which are just ordinary cross-border credit-card transactions, or they might have a payment processor in the other nation - and they might make money by selling support-and-services instead of licenses (e.g. Canonical). These kinds of "exports" are very difficult to impose trade-barriers on without scrutinizing every cross-border transaction (or just impossible if you consider technologies like Bitcoin).