This category has been created for certain fungi with a flat, sheet-like fruitbody from which abundant and extensive string-like growths extend outward. Those 'strings' are called cordons or hyphal cords and you can see an example here - http://www.anbg.gov.au/fungi/images-captions/phanerochaete-sp-0190.html. The cordons can extend for many centimetres.
The fruitbodies appear on the lower surfaces of wood that is lying on the ground and colours range from whitish through yellow to brown - with yellow the most common colour seen here.
The sheet is two-layered. The surface is more-or-less continuous and smooth (perhaps with scattered pimples or warts) but with age or in dry conditions it may crack. Between that surface (or hymenium) and the wood is the subiculum, a fibrous layer (a bit like cotton wool) and if the surface is cracked you can see the underlying subicular fibres through the crack. The subiculum develops first, so at the margins of the sheet (the youngest part of the fruitbody) you have only a fuzzy subiculum, with the hymenium starting a little way in from the margin. The subiculum may be the same colour as the hymenium or not. In the latter case you find a white subiculum with a coloured hymenium and, as the colour develops with time, you often find a white hymenium just behind the fuzzy margin and the colour a bit further inward.
For a long time Phanerochaete included many of the species with cordons. In 2004 Greslebin et al (see the references) defined the closely related genus Rhizochaete and one of the macroscopic differences noted by those authors was the reaction to potassium hydroxide (KOH). They noted that in Rhizochaete both the hymenium and the cordons turn red or violet in KOH, not so with Phanerochaete (where there may be no reaction at all or a reaction in one of hymenium or cordons but not in both). Nakasone et al (see the references) added some more species to Rhizochaete and slightly emended the definition of the genus, to allow for a few new species that showed no red/violet KOH reaction.
Asterostroma and Vararia fruitbodies are creamy or brown and may have cordons, but would generally not as abundant and extensive as in Phanerochaete/Rhizochaete.
There are sheet-like fungi that are two layered and so have have a fuzzy, subicular margin (but no cordons) and others that have cordons (but are not two-layered or where the hymenium is not smooth).
Greslebin, A., Nakasone, K.N. & Rajchenberg, M. (2004). Rhizochaete, a new genus of Phanerochaetoid fungi, Mycologia, 96, 260-271.
Nakasone, K.N., Draeger, K.R. & Ortiz-Santana, B. (2017). A contribution to the taxonomy of Rhizochaete (Polyporales, Basidiomycota), Cryptogamie, Mycologie, 38, 81-99.