Below is a summary of seedling stock types that I cut and pasted from the state DNR site.
Credit to WA DNR Website. Their procedures for their product. Different Nurseries may have a slightly different protocol
Seedling Stock Types (Summary)
This term designates a seedling grown for one year in a seedbed, harvested, root pruned to five inches and transplanted back into a nursery bed at approximately six seedlings per square foot. The transplanting process results in a larger caliper and a more fibrous root system. The seedling will have more side branching, with a minimum of 10 inches in height and 4 millimeters in caliper Douglas-fir 1+1 and 2+0 (stem diameter at the root collar). The root system on a 1+1 plus the extra storage of food in the caliper and root system will allow the seedling to survive on an infertile site, compete with other vegetation and give it a better chance of surviving browse damage.
This designates a seedling that was grown at approximately 25 seedlings per square foot in the seedling bed and grown in the field for two years (never transplanted). After two years the seedling is ready for out plant. The production costs are low because the seedling had not been lifted, packed and transplanted as in all of the transplant stock types. The root systems on such stock type are pruned horizontally in the ground at a six-inch depth and vertically between each row at the end of the first growing season. These cultural activities encourage branching of the remaining roots and promote more fibrous roots required for out planting survival. This stock type will survive in a site that has low competing vegetation and minimal levels of animal browsing.
Plug (P or S-8)
This is a seedling grown in a greenhouse in containers that are narrow and deep. For some species, this stock type reduces the time between request and out plant. For some species, growing plug stock type is necessary due to low germination and early growth. Various sizes of containers are available but the target is a styro-2A (two cubic inch containers) if the seedling will be used for a Plug+1 stock type. A styro-10 will be used if the seedling will be used for out planting.
After growing in the greenhouse for a year, the seedling is extracted from the container, root pruned at 5 inches and transplanted in a nursery bed at approximately six seedlings per square foot. As with the 1+1, the root pruning and transplanting generates a larger caliper stem and more mass in the root system. Cedar, hemlock, larch and some species of pine and true firs are propagated as Plug+1.
Like the P+1, this stocktype combines greenhouse and bareroot phases, but the process is completed in one year instead of two. Seedlings are started in late winter in the greenhouse in small containers (2 cubic inches or smaller). In early summer, seedlings are transplanted into nursery beds at approximately six seedlings per square foot. The early start and low transplant density allow for the production of a relatively large seedling in a short amount of time.