Analysis
Property
Also called measurement property.
Property is the container that holds one site or app's measurement stream. It has its own id, its own settings, and its own data.
How it is measured
A property is identified by a measurement id embedded in the tag. Everything that tag collects goes into that property. Settings such as time zone, currency, and data retention are set at this level.
Verify by loading the site and checking that a hit arrives in the expected property. Confirm the id on the page matches the one in the admin settings.
Worked example
A boutique hotel runs its marketing site and booking engine on separate domains. Both send hits to one property, so booking page sessions appear as if they were visits to the marketing site.
The owner splits them into two properties and keeps a shared cross-domain setting for the handoff. Booking sessions of 1,240 a week now appear in their own place instead of inflating the marketing number.
How it differs
A property holds data for one site or app. An account hierarchy is the whole tree of accounts, properties, and views. The property is the unit you point a tag at.
Common errors
Using one property for many unrelated sites. Setting the wrong time zone. Copying a tag with the wrong id. Not recording the id in the project docs. Changing currency after launch.
In practice
Write down each property's id, domain, time zone, and currency in one place. Verify the id on live pages. Create a new property rather than reusing one for a different site.