Well I go with the following:
full sun is anytime of day with 6 or more hours.
Full shade gets no sun at all.
Part shade gets morning or evening sun but no sun from noon or 1-ish till late afternoon...In other words Shade for the hottest part of the day.
Dappled or filtered sun, anytime of day, usually through trees or trellis...may get some morning or early even sun.
This has worked for me....others may have different theories that they use.

Sierra

Anything less than 6 hours of sun is considered shade

Anything with at least 6 hours to 8 hours of sun is considered semi-shade and

Anything more than 8 hours of sun is full sun.

Now some plants are sun hogs and need sun all day so they need the full 15 hours. These mostly originate from very hot countries.