Categories are artificial.
Categories are parameters that were created by us, for us. We create categories to help us organize information. Categorizing helps us make sense of large amounts of information. It is a cognitive mechanism that has allowed us to evolve as a species. Thousands of years ago, it was vital that we could differentiate the onslaught of incoming stimulus so that our attention wasn't overwhelmed. Instinctively we continue to do this. When we do so, we choose some criteria that we can identify as a basis for the categorization and then categorize things on the differences among this criteria.
After information is categorized, we now have a framework where we can now compare and contrast it to the information in other categories. When similarities in one category are discovered we develop assumptions about all others in that category. This is quite effective for dealing with most simple phenomena and prevents us from wasting our time and energy on everything that we encounter. Problems arise when we attempt to categorize things that are complex and have many individual differences.
Tea is simply one of these complex things.
Korean green tea is generally put into three categories- Ujeon, Sejak, and Jungjak (spellings vary).
Jungjak are the tea leaves that are picked between May 20-30th, they contain a larger leaf. Sejak is picked between April 30th-May 10th, contain a smaller leaf. Ujeon is picked on or before April 20th, these leafs are the smallest of the teas in Korea. These categories were traditionally based on when the leaf is picked in China. These dates correspond to the lunar spring calender.
There are some issues with these categories party because Korea is much cooler than China. If Korea has a late winter, than the picking dates will get pushed back. Also, there are gaps in the picking times (May 11th-19th and April 21st-29th), clearly Koreans don't just take a holiday in the middle of prime tea picking season. In a way these dates just pay homage to the dates used long ago, they are not in anyway taken seriously in Korea today. The tea is picked when the tea needs to be picked.
Quality is often connected to these categories. Ujeon, being the highest quality, next Sejak, followed by Jungjak. Although I have personally never tried a Ujeon that wasn't beyond excellent, often exceptional Sejak, and Ujeon teas from one maker are preferred over the Ujeon of another. All teas have their own individual differences, subtleties, flavors, that influence the end product. The tea drinker also has his own preferences and tastes when it comes to tea. In the end, these categories heavily influence the price as Ujeon usually fetches high prices, and of course Jungjak, a much lower price. This of course doesn't necessary mean that some Jungjak tea isn't great.
Things are what they are.
Categories are only artificial, created by us.
Great teas are only extraordinary creations by us.