Seed Pumpkins are pumpkins whose food value is the seed in them, rather than the flesh of the pumpkins. The focus is the overall weight and quality of the seeds that can be harvested from the pumpkin: it is not size of the pumpkin, nor the weight of its flesh, so much as weight of the seeds that can be harvested from it.
In fact, the flesh is usually coarse and stringy with no taste, and not worth trying to eat. It can, though, be fed to livestock or used as compost.
Squash are included in the category of “Seed Pumpkins” for convenience when they are a variety of squash grown primarily for their seeds.
Seed Pumpkins will almost always have one particular trait in common: they produce hull-less or “naked” seeds. What this means is that they lack the tough, white shell (aka seed coat, aka “testa”) that is typical of most pumpkin seeds. They do in fact have a hull, but it’s a soft one, not a hard one. What causes the lack of a tough shell is “reduced amounts of lignin and cellulose in certain cell layers with the seed coat.”   The soft hull will usually be mostly invisible, or translucent, causing the seeds to look green, because you are seeing the actual kernel of the seed through the barely-there hull.
The types of seeds desired from pumpkins can be broken down into two types: confectionery seeds, for snacking, and oil seeds, for pressing into oil.
Pumpkin Seeds for Oil
Hull-less pumpkin seeds first developed in the late 1800s in southern Austria. There was a tradition in the region of pressing pumpkin seeds for oil prior to that, but it was pressed from regular pumpkin seeds which had to be hulled first.
Hull-less pumpkin seeds are more desirable than hulled ones because less heat and pressure is required to extact the oil, which results in a better quality oil.
It takes about 2.5 kg (5 1/2 pounds / 18 cups) of pumpkin seeds to make a 1 litre (1 quart) bottle of pumpkin oil. Consequently, it is hard to grow enough oil seeds in a home garden to produce enough oil yourself to make it worthwhile, and it may be better to treat such seed as snack seeds. Almost all pumpkin seeds that are good for oil also make good snack seeds.
Pumpkin Seeds as Confectionery Seeds for Snacking
Pumpkin Seeds as a snack food are particular popular in the United States. They are sometimes sold labelled as “pepitas”, which is in fact the Mexican name for them.
They can be eaten raw or roasted, out of hand. Other uses include as a topping for muesli, other cereals, yoghurt, salads, soups, in muffins, in trail mixes, or in smoothies (grind first on their own.)
To store pumpkin seeds for any length of time, they need to be roasted or dehydrated first.
See separate entry on Pumpkin Seeds for a full discussion of pumpkin seeds as a snack food.
When harvesting pumpkin seeds out of a pumpkin, it is best to cut the top off the pumpkin and dig them out from there, rather than to cut the pumpkin in half, as that can damage many seeds. The seeds are best simply dug out with your hands, using your fingers as a “rake.”
Rinse and work off any flesh clinging to them.
If you are going to roast or dehydrate them, some people advise to soak them first in salted water overnight.
To roast them, drain well, then toss with an oil such as olive or sesame, and with any seasonings or salt desired. Spread them out on a baking sheet only one layer deep. Roast at 120 C (250 F) for about 45 minutes or until they crisp up; mixing every 15 minutes. Store in a sealed container for up to two weeks.
To dehydrate pumpkin seeds, toss with any seasonings or salt desired, but no oil. Roast at 65 C (150 F) for 8 to 12 hours, mixing every one or two hours, until crispy. Store in a tightly sealed container for up to a few months; longer in refrigerator.
1 cup hull-less pumpkin seeds = 140g = 5 oz by weight
 Yi-Hong Wang, Tusar Kanti Behera, Chittaranjan Kole. Genetics, Genomics and Breeding of Cucurbits. CRC Press, 2011. Page 105.
 “This is because there are still actually seed coat layers in hull-less cultivars. There are five seed coat layers in the testa of a ripe hulled seed namely, epidermis, hypodermis, sclerenchyma, arenchyma and chlornchyma (Zraidi, Pachner and Lelley 2003). The mature seed coat is tough and leathery in hulled pumpkin seed cultivars but is reduced ‘to a thin, usually greenish membranous-like cover’ in the hullless cultivars (Loy 2004, p. 353). …In cultivars classed as completely hull-less, the outer seedcoat layers are reduced to the extent that the seed takes on the colour appearance of the inner seedcoat layer which is normally dark green (Loy 1990, p. 404).” — G.G. Baxter, K. Murphy and A. Paech. The Potential to Produce Pumpkin Seed for Processing in North East Victoria. Government of Australia, Rural Industries Research and Development Corporation. RIRDC Publication No. 11/145. February 2012.
Loy, Brent J. Hull-less Seeded Pumpkins: A New Edible Snackseed Crop. In: J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press, Portland, OR. 1990. pp 403-407.