A tag can be applied to any geometry object (line, rectangle, circle and compositegeom*), and specifies additional properties of the object.

The tags you can apply are fixed, but you can have more than one by using a comma separator, for example walkable,detaching.

What do the different tags do?

Pops any container ball and releases its children. Container balls are those that have a contains attribute. The existing ones in the game are: Beauty, Ugly, ZBombMom and Bit ("contains" a Pilot).
Can be destroyed by bombs, Bits & PixelProducts & ZBombs, a subtype of break=X.
Can be destroyed by bombs, Bits & PixelProducts but not ZBombs, a subtype of break=X.
Specifies the "damage" needed to break this object. If a Goo Ball explodes, it deals "damage" to all geometry within the explosion radius, linearly scaled from the explosion force at the center to 0 at the edge. If the center of a breakable geometry object takes enough damage in one hit, it breaks. NOTE: This experimentation was done after the latest version of WooGLE was released, you you need to modify it by hand.
Kills all balls on contact (* see note on compositegeom below)
Kills all balls except those with the attribute invulnerable=true (Bone, Bombs, and Anchors)
(* see note on compositegeom below)
Detaches any ball from a structure, even the un-detachable.
Destroys non-static geometry objects on contact. Note: If 2 geomkiller objects come into contact with each other, only one will explode - which one seems consistent but "unpredictable"
Removed in v0.70 (**see below)
Balls that make contact with this will reverse direction, so it's effectively a "stop sign" indicating that they should not walk off a cliff etc. This has no effect on thrown, flying, jumping, falling or climbing balls. Note: If a ball falls into a stopsign area, and lands on a walkable surface it will be thrown (violently) to the nearest edge of the stopsign shape, even if that's straight down into (inside) the floor.
Balls will not walk or roll on this surface, they will just sit still.
Balls will walk or roll on this surface (default if tag field is left blank)

Balls cannot be dragged within this volume. Automatically becomes non-solid. Discovered by MyGod long after Daft as Brush left, so it needs to be added in by hand in WooGLE.

* Notes about compositegeom objects and tags

  1. Only the tag applied to the compgeom itself has any effect. Tags applied to child items are ignored.
  2. The game doesn't process the deadly tag 100% correctly on compositegeom objects. This gets a bit complicated...

    When a compgeom is tagged as deadly it does not kill "sticky" things.
    StickyBombs & AnchorStickys never die. Pokeys will die if they just fall into it, but survive if they are trying to attach.

    Tagging as mostlydeadly instead will always kill Pokeys and most other balls, but won't kill Boney, Bombs and Anchors.

    Tagging it as both, deadly,mostlydeadly, makes it kill everything except StickyAnchors and StickyBombs.

    The Solution! In addition to the deadly or mostlydeadly tags, also add detaching. This makes a compositegeom act exactly the same as any other geometry.. deadly kills everything!

** Note on kindasticky

This value attribute is present in one of the original game levels, but it looks to be there by mistake. In Second Hand Smoke the main platform is tagged kindasticky,walkable, however the kindasticky has absolutely no effect.

There is, however, a material called kindasticky, which is never used in any of the original levels. Most likely, 2D Boy meant to set the material of the platform to kindasticky, but accidentally put it in the tag field instead.