Answer: These terms are always a bit subject to interpretation, you are right. Sometimes the gardener has to experiment and see how things do by trial and error especially since other factors such as drainage, wind and soil can also affect the growth of plants.
In general, full sun would mean full sun all day long or possibly six or seven hours of sun but including the hours of noon. Afternoon sun is hotter than morning sun, so a plant needing partial shade might tolerate morning sun but would not be likely to tolerate hot afternoon sun. Conversely, a plant requiring full sun would be more likely to do well in a half day of afternoon sun than it would in just morning sun. Partial shade can also mean dappled light all day. The shorter the period of dappled light then the closer you approach to full shade; full shade may also be encountered in an area shaded all day by a building.
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