Good armok man! Consult the page on standard tiles in ANY SOURCE BOOK EVER! _ is the standard void (stand-able) tile. It's also one of the few tiles that *isn't* supposed to be in the legend (as per the memetic safety standards set out in the overhaul during the communist conflict update).
Deep breath. There's a few tiles that we never list in the legend, amongst them are void tiles, (because in normal play, it's important to differentiate between the void tiles that are there and the non-tiles beyond the void tiles) basilisks, (which are represented as a rooster in case of someone using a graphical tileset, although there was a large push that gt users deserved it) pro-RIAA types, (in case they're copyrighted) sigils of madness, explosive runes, holes magiced into bottomless (as differentiated from normal tiles that don't have a bottom, magiced bottomless holes have the effect of making people want to jump into them, once again, this is for graphical tileset safety, as laid down in the guidelines for making tacticus more user friendly (cutting down the learning cliff: safe tile set rules (which is really kinda a crappy supplement, but it also contains the rules for mounting bayonets on heavy weapons, so we have to use it (especially because the fire imp couldn't use any sort of bladed implement without the abuse of the bladed flamethrower rules(otherwise, it'd have problems melting the blade(or summoning the archangel Michael(thereby triggering Abhamaic rules (and a flamewar))))))), and games of tacticus.