From The SEA.server forums FAQ. pages. I hope both NA and EU follow suit
OK to use mod functionality:
Not forbidden for now, but may change in the future:
- Mods that reveal players’ additional equipment.
- Indicators pointing in the direction of currently spotted enemy tanks outside of actual field of view (ie. not shown on the player screen), with names and distance.  This is only legal if there’s only one arrow pointing to the nearest enemy tank.
- In-battle camouflage rating calculators
- HP Pools (adds information about HP of the tanks (ally and enemy) as last seen by the player)
- Battle Assistant: provides alternative view for SPGs
- Extended 6th sense sound duration, eg prolonged to 10 seconds.
- White dead tanks: makes destroyed tanks more visible by making them totally white
- Modifications to objects to make them more visible, like bright yellow trains etc
- Zoom out mods
- Enemy gun indicators on the minimap: shows direction where enemies within the draw distance are pointing their guns (on the minimap)
- Fog removal
- Sound mods
- Effect mods: changing how explosions look like, removing smoke etc
- Server-side reticle: secondary aiming point that corresponds to where your tank is aiming on server side
- Damage indicator mods: OK as long as these don’t show the enemy reload time.
- Reload time indicator (approximate with no countdown). Example.
- Clown skins and skins that show locations of crew and modules.
- In-game penetration indicator mods.
- Extending both the sound and icon visibility duration of 6th sense.
- Received hit indicators. Example.
Thanks to DoM1n for the heads up.