trigger_add_or_remove_tf_player_attributesI will be playing around with this A LOT and I'll post anything interesting here.
This is a brush entity that currently doesn't actually exist in the fgd of tf2 but whatever it still works.
From what I can tell so far, this is basically an in map, vanilla version of the tf2items sourcemod plugin. But I doubt that you'd be able to create your own weapons with this.
Attributes are the modifiers attached to weapons or players, and their effects can range from modifying jump height or max health to swapping out projectile type (rocket minigun, anyone?) or doing this ("torso scale"). From here on, I'll refer to an attribute attached to a weapon as a "weapon attribute", and an attribute attached directly to a player as a "player attribute".
It's important to note that attributes usually have nothing to do with "conditions".
Weapon attributes are pretty clear-cut. Player attributes are used for things like the MVM Player upgrades, set bonuses (those still exist), Wheel of Fate effects (sometimes), and more recently, the Dalokohs Bar effect (which used to be a weapon attribute until Gun Mettle exploits forced them to rework how it did its thing). What's different about the Dalokohs Bar is that the +50 "hidden maxhealth non buffed" it adds has a timer associated with it, in a special type of player attribute that Valve has taken to calling "custom attributes". This new trigger adds "custom attributes" in the same way the Dalokohs does, with an attribute name, value, and duration.
-FlaminSarge
I haven't played around with it much yet. But it looks like the only attributes that will work with this are stuff that are specific to either 1 weapon slot (max secondary ammo) or generic stuff like health or regen. Or a better way to say that is I don't know that you can apply these attributes to select weapons.
How to use it:
Now because this isn't actually in the game you will need to put all keyvalues in yourself. That is until someone writes up an addition to the fgd.
More Info:
http://tf2maps.net/threads/trigger_add_or_remove_tf_player_attributes.26677/Prefab:
https://www.dropbox.com/s/x26hzhnbjlvlfe6/500_maxhealth_noOverheal.vmf?dl=0