Here's the wiki entry for it: https://geckwiki.com/index.php?title=SetLeveledListFlags
The command sets whichever flag you specify with the number after it, and using the number 1 tells it to set the first of the three values, which is all >= player level. I use this flag to add things to lists like raider weapons, allowing me to give them higher tier weapons as the player's level increases.

The base behavior for a leveled list with no flags is:
When the leveled item is grabbed, it will choose the option that aligns with the player's level, and give as many copies of that item as are referenced by the leveled list call. For example, if you have this list:
https://imgur.com/Fj14PpK

If a character or container asks for three of this list, it will first check what level the player is and decide it's item. If the player's level is less than 10, it will choose either a laser rifle or laser pistol at random, then provide three rifles or pistols. If the player is level 10+, but not 20+, the item would be a plasma rifle, and the container or npc would have three of them. If their level was above 20, it would choose a tesla rifle every time.

If you add the Calculate for each item in count flag, it means that while the player's level is below 10, each individual item is calculated separately. If you had the same container with 3 of this leveled list with this flag set, the container would have three total items, but it might be 3 pistols and no rifles, 2 pistols and 1 rifle, 1 pistol and 2 rifles, or 3 rifles. If the player's level is above 10, it will still only choose the plasma rifle.

If you add the Calculate from each level <= Player's level flag, with a player above level 10, the game might choose a laser pistol, rifle, or plasma rifle. However, if this flag is set but Calculate for each item in count is not set, you will still see three copies of whatever weapon the game chooses in the container.

The Use All flag is very simple, it just provides every item in it's list for every count that's called. If you set the Use All flag on this leveled list, the container would have three copies of: laser pistol AND laser rifle AND plasma rifle AND tesla cannon, for 12 items in total.


Calculate for each item in count is often used for condition lists, to allow the player to see higher health drops of items at later levels.

Calculate from all levels <= player's level is frequently used for things like weapon pools, like LL2Tier2GunsMercenary.

Use All is usually used for things like WithAmmo lists, that include a Condition list for a weapon, as well as a single bullet, an amount of 75% chance of the same ammo, and a 75% chance of casings. This provides npcs with everything they need to use their weapons.

I usually add the All levels <= player's flag to lists that weren't intended to be used as leveled pools in the base game, but that I want to add higher tiers to. For example, Brotherhood0WeaponGun is a single withammo weapon list. I want to add another weapon option once the player is at level 10 or whatever, so I set the flag by script and then I can freely add the other withammo list at whatever higher level I want.