Lots of terms exist to describe the way sailing vessels were equipped over the millennia. Old terms get reused, and sometimes the terms we use aren't specific.
Sailmakers refer to sails by the length perpendicular (aka the LP), not the area. If the distance from the luff to the clew = J, it's a 100%. So the percent = the measured LP as a percent of J. That's about as specific as you can get.
From
https://sailingfortuitous.com
100% headsails do not necessarily fit inside the foretriangle. If the clew is higher than the side deck, it will overlap the mast at the clew.
Below: from
https://www.uksailmakers.com/encyclopedia/4-9-jib-top/
On a modern marconi rig, the foresails are sails that are hoisted in front of the mast on a forestay . I think that headsails are sails hoisted on a headstay on a masthead rig. But everybody uses the two terms interchangably.
In contemporary parlance, a working jib usually has an LP between 100-115%. The clew overlaps the mast a little and overlaps the spreaders too.
For fractional rigs with swept spreaders, the "non-overlapping" headsail will, in fact, overlap the mast, and will probably be around 105% LP or so. (as JackDaw said). But because the spreaders are swept aft, it can be sheeted in very tightly without hitting the spreaders. It overlaps the mast, but not the spreaders.
I usually call a sail with an LP greather than 115% a genoa.
The number 1 sail is simply the biggest foresail in the boats inventory. For some boats, it's a 150% LP but it could be larger. The luff is usually as long as possible so we call it a "full hoist". (Usually it's about 95% of the hoist, due to practical considerations. )
The number 2 is the next LP down. That might be a 125-135% LP. The luff is on a 135% #2 is usually a "full hoist." (ie 95% of the available hoist)
The number 3 is the next one down in size. Usually a 100 - 115% LP, and sometimes doesn't take up the full available space for the luff. Often the luff length will be only about 80% of the full hoist on a masthead rig, but the luff on a fractional rig will be 90-95% of the full hoist.
The #4 is often an 70-80% LP, with a hoist about 80% of full hoist. That's a heavy air jib, but not a storm jib.
For many cruisers with roller furling, a 135% LP with a full hoist is their #1 Foresail. The 150% flogs a lot when tacking so it gets worn quickly. It's a lot of work to tack. The 135% furling genoa for a cruiser is a practical sail, because it's easy to tack, and it works well if you furl it to a size that balances with a double reefed mainsail.
If you're a PHRF racer, you want to have lots of different foresails, because there's no effect on your rating if you have a #2, 3, and 4 in addition to your #1 genoa (which is usually a full hoist 155%) .