Sanctions are announced by governments and enforced largely by private institutions that must interpret them. The gap between the announcement and any practical effect is where most of the difficulty sits.
An announcement is an instruction to intermediaries
A measure names people, companies, vessels or whole sectors and prohibits dealings with them. It does not, by itself, stop a single transaction.
The prohibition binds everyone inside the issuing state's jurisdiction, which in practice means banks, insurers, shipping agents, exporters and payment processors.
Those firms then have to work out whether a given customer or cargo falls inside the measure, which is a screening problem long before it is a legal one.
Banks carry most of the screening load
Payments are the chokepoint, because cross-border trade settles through a comparatively small number of correspondent banking relationships.
Banks run names and addresses against lists, and matches are rarely exact. Transliteration, common surnames and generic company names all produce false hits that a person must clear by hand.
Since the penalty for processing a prohibited payment dwarfs the cost of refusing a legitimate one, banks tend to over-block, and lawful business in the same region gets caught in the sweep.
Ownership rules blur the perimeter
Most regimes extend the prohibition to entities owned or controlled by a listed party, usually above a stated ownership share.
That turns compliance into a corporate genealogy exercise, because ownership can be layered across several jurisdictions with registries of very different quality.
A firm that is not itself named can therefore be off-limits, and a counterparty may not know that its own ultimate owner has been designated.
Physical trade adapts faster than finance
Cargo can be re-routed through a third country, relabelled, or sold to a trader who resells it onward without asking where it ends up.
Enforcement responds by watching for trade patterns that make little commercial sense, such as a sharp rise in shipments of a specialised component to a country with no industry that uses it.
This is why measures are so often widened to cover intermediaries, freight forwarders and shipping insurance rather than simply tightened on the original target.
Penalties and licences do the steering
Agencies do not police every transaction. They pursue a small number of cases, settle them publicly, and the size of those settlements sets the caution level for everyone else.
Alongside the prohibition sits a licensing system that permits specified flows to continue, commonly food, medicine and humanitarian payments.
Licences are what keep a sanctions regime from hardening into a total embargo, and arguments about a regime usually concern which flows should be licensed rather than whether the measure should exist.