Sanctions are restrictions on economic and financial dealings, imposed to change behaviour without using force. They run from freezing one person’s bank account to prohibiting most trade with a country.
The three things that determine whether a rule applies to you
Who imposed it. A US measure binds US persons and, in many programmes, transactions touching the US financial system. An EU measure binds EU persons and conduct in EU territory. A UN measure obliges member states to legislate, and it is the national legislation that binds you.
What it restricts. Some measures prohibit dealings with named parties. Some restrict particular activities in a sector. Some restrict goods by their origin. Some restrict services. These are different questions with different answers, and screening a counterparty against a list answers only the first.
Where you are. The same transaction can be prohibited for a bank in New York, permitted for its client in Dubai, and prohibited again for that client’s insurer in London.
That last point is the one that generates most of the material on this site. Almost every technique described here exists to exploit a seam between two of those three variables.
Why the money matters more than the law
A restriction is only as effective as the chokepoint it can be enforced at. The reason US sanctions have outsized effect is not that US law is unusually strict but that most international trade settles in dollars, and dollar clearing runs through institutions within US jurisdiction.
Understanding that explains what follows: why payment routing is such a central technique, why the price cap works through insurers and shipowners rather than through customs, and why so much evasion effort goes into keeping a transaction out of a particular currency.