Use this tag to identify that you're using the Enterprise supported distribution of Liferay starting with Version 7.0. Also tag with the appropriate version, e.g. "liferay-7.2".
Liferay DXP stands for "Digital Experience Platform" - it is the enterprise-supported version of liferay-7, liferay-7.1 and liferay-7.2. In addition to the support is also offers extensions that are useful in the enterprise environment. Use this tag to identify that you're using the enterprise supported release, and remember to also tag the exact version that your question relates to.
The implementation is highly modularized, utilizing osgi.
More information in the more basic and general version-independent tag liferay that can easily go along with this tag.
"Liferay DXP 7.0" was initially released as "Liferay DXP" but later rebranded with the version information in the name (as newer versions were released). This tag has started as being limited to "DXP 7.0", but has been used independent of the exact version (7.0, 7.1, 7.2) for a long time. The updated tag description acknowledges this by asking for the version to be tagged additionally.