Specified by Types/string
Slash (/) is always used as directory delimiter.
The path should always begin by the specification of the root.
Path starting with __core__ will access the resources in the data/core directory, these resources are always accessible regardless of mod specifications.
filename = "__core__/graphics/arrows/underground-lines.png"
Path starting with __base__ will access the resources in the base mod in data/base directory. These resources are usually available, as long as the base mod isn't removed/deactivated.
filename = "__base__/graphics/entity/basic-transport-belt/basic-transport-belt.png"
The format __<mod-name>__ is placeholder for root of any other mod (mods/<mod-name>), and is accessible as long as the mod is active.