Yield is the number a listing most wants to impress you with and the one it can least promise. It records what a variety managed under someone else's light, in someone else's room, with someone else's skill, and none of that comes with the seed.
What the labels mean here
Shops describe yield in words rather than grams, and they choose the words themselves. In this index those words fall into four groups: 101 strains are marked heavy, 68 large, 79 XXL and 26 medium.
No standard sits behind any of them. One breeder's "large" is another's "XXL", and nobody measures the two against each other. Read them as a ranking inside one shop's catalogue, where they mean something, and not as a scale across shops, where they do not.
What actually decides the number
Light, ahead of everything else. Yield follows the total light a canopy receives more closely than it follows any other input. The working measure is daily light integral: the photons delivered per square metre per day. Flowering cannabis makes use of roughly 35 to 45 mol/m² per day at ordinary CO₂ levels. Push the intensity past about 1,000 µmol/m²/s without adding CO₂ and you get bleaching rather than flowers.
In practical terms: a lamp suited to the area, hung at the right height, running the full schedule. Everything else on this list comes second to that.
Canopy area, not the number of plants. Two plants under a good lamp will fill the space and produce what the space allows; six of the same under that lamp produce the same total in smaller pieces. Training, whether by topping, bending or a screen, spreads a plant across the lit area; it does not make the plant bigger.
Time. A longer vegetative stage builds a bigger frame to flower, and a photoperiod plant leaves that choice to you. An autoflower does not, and that is the real reason it usually yields less per plant.
Keeping the plant healthy. Overwatering, pH drift and mould take more off a harvest than any technique will ever add to it.
Grams per watt, and why it is a trap
The old benchmark is grams of dried flower per watt of lighting. Around 1.0 is respectable for an efficient LED setup, and anything well above it comes from commercial rooms with CO₂ and full climate control. It works as a sanity check and fails as a target, because it rewards under-lighting a space, and because a watt describes what a lamp draws rather than what it gives off. Two lamps of the same wattage can differ by a third in usable output.
Reading yield on this site
The label is shown per strain as the shop stated it, alongside flowering time, height and price per seed. Where a variety is described as heavy, that is the breeder's own claim about their own conditions.
If yield is the priority, the more reliable signals in a listing are flowering time, because a longer flower usually means more of it, and height class, because a plant that fills the space produces what the space can support. Both are covered in height, stretch and space, and both are stated per strain rather than summarised in a word.