This section explains how to deal with the many available new graphics files. Some of them can't be loaded at the same time or aren't available in all climates.
Most of the new graphics sets are designed for a certain climate. They will only work in the right climate, for example you can't use the DBset in anything but the temperate climate. The DB set is also incompatible with the Temperate set ("tempset"), which means only one of the two can be active at the same time.
The most important TTDPatch switches that control activation and automatic selection of graphics are "loadallgraphics" (only applies in 2.0) and "saveoptionaldata".
If "loadallgraphics" is off (which is the default, and the only state in 2.5 and later), only those graphics are loaded that were loaded when the game was saved. If you later add new grf-files to newgrf.cfg, those graphics won't be loaded in your existing savegames. The reason for this is that the new graphics also change the vehicle stats, and could lead to an inconsistent state of your vehicles. If you set the switch to on, all new graphics will be loaded when you load an existing savegame. This switch only applies to new graphics, so it should better be called "loadnewgraphics", but for historical reasons it isn't.
The "saveoptionaldata" switch allows TTDPatch to actually save the information about active graphics with your savegames. It is recommended that you always leave it turned on. If turned off, TTDPatch doesn't know which graphics are old and which ones are new, so it assumes all of them are new. In that case, "loadallgraphics" applies to all entries in the newgrf.cfg file.
Additionally, you can use the Cht: Graphics sign cheat to activate or deactivate individual graphics files. See the description of that cheat to find out how it works.