From Quake Wiki

Specifies that the 'effectinfo' namespace is special. Specifically that the engine is able to parse DP's effectinfo files (and without additional user configuration) by the mod prefixing any effects, for eg: "effectinfo.myeffect".

For greater compat with DP, it is generally recommended to just call particleeffectnum("effectinfo.") to force the client to load particles from the specified namespace. Then, eg particleeffectnum("myeffect") can automatically use effects imported from the effectinfo.txt file, in much the same way as DP.

Implementation difference: DP doesn't network any effect names. This means glitches if the effectinfo files differs even slightly. In contrast, FTE DOES network the effect names, like any other precache. This means that you really should be precaching these effects otherwise you risk the late-precaches getting stuck in the reliable message stream while the unreliable messages go straight through. Or in other words, precache particle numbers properly or they might fail to spawn the first time(s) any specific effect is used (on account of the client not having a clue which effect to use/load. Note that this extension gives no real guarentee about compatibility, only that the engine can try to load them. Compatibility may be improved in time.