As Minetest world is virtually infinite, only part of it can be loaded in memory at any given time. Actually, there are 3 possible states for a mapblock:
- Unloaded
- Loaded (cached)
- Active
Minetest activates blocks around players, and also so-called “force-loaded” (“anchored”) blocks. All internal computations (like liquid flow) happen in active blocks only. But active blocks are expensive, so the activation range is very limited. Blocks farther than that may be loaded (to support larger view range), but they are not activated until you come close enough.
For most mods, that just means they will work near players only. But in Mesecons it may easily happen that a device is very large and spans many blocks, only some of which are active. Such situations need special treatment.