For loop isn't spawning parts as intended

[goldenstein64 (Shoop)]

You could probably just index each of your arrays with the index already provided from default_bunker_materials:

for i, v in pairs(default_bunker_materials) do
	local part = Instance.new("Part")
	part.Name = v

	-- new block:
	part.CFrame = default_bunker_terrain_position[i]
	part.Size = default_bunker_terrain_size[i]

	part.Parent = terrain
end

If you are wondering why I left out the parent argument from your Instance.new("Part"):

PSA: Don't use Instance.new() with parent argument

I’ve discovered a pretty bad performance issue in one of top games that has to do with Instance.new and wanted to write about this, since this is not obvious unless you know the system inside out. Tthere are several ways to create a ROBLOX object in Lua: local obj = Instance.new(‘type’); fill obj fields local obj = Instance.new(‘type’, parent); fill obj fields local obj = util.Create(‘type’, { field1 = value1, … }) If you care at all about performance, please only use the first option - I wi…

If this works, why this works

Whenever you use an array, you are secretly indexing them with numbers starting from 1:

-- this
{"a", "b", "c"}

-- is the same as this
{
	[1] = "a",
	[2] = "b",
	[3] = "c"
}

So this gives the i meaning in the for loop by giving you the number part on the left, which every other array has as well:

local array = {"a", "b", "c"}
local otherArray = {"x", "y", "z"}

for i, v in pairs(array) do
	print(i, v)
end
--> 1 a
--> 2 b
--> 3 c

for i, v in pairs(array) do
	print(otherArray[i])
end
--> x
--> y
--> z