IR Systems

Think of it this way. Digital cameras are generally made by companies that do other consumer electronics. So, the first wireless controls for off-camera flash that were built were based on existing IR* remote technology—like how your TV remotes work. This has an obvious advantage for manufacturers of not requiring radio bandwidth allocation in all the countries where they want to sell their product. Optical/IR transmitters and receivers communicate by a series of light pulses, and how those pulses are interpreted are typically by a brand-specific protocol. Third party flash makers, like Nissin, et. al. reverse-engineer this protocol to make flashes that are compatible within those systems.

And your 60D can use its pop-up flash as your transmitter. The basic limitation of using the pop-up vs. an on-camera speedlight to be the optical transmitter is that the pop-up can't be rotated or tilted, has a shorter range, and can't communicate high-speed sync. So, it can limit where you can place your remote flash, and you have to use a shutter speed at or below your camera's sync speed.

Like a TV remote, you have to point your transmitter at your receiver or bounce the signal off of surfaces for things to work. Using the pop-up as your master and placing your flash behind you, or trying to hide the flash behind a door, or use it outside a window to fake sunlight--these are all relatively problematic with an optical system, but easily accomplished with a radio system. And TTL-capable radio triggers do allow for remote power control, which becomes extremely handy if your flash is someplace inaccessible or you don't want to have to keep ripping open your Westcott Apollo softbox to get to the flash to adjust it. But you do have to worry about RF interference, since to get around bandwidth allocation issues, most of the newer radio triggers use the 2.4 GHz worldwide ISM band, which is crowded with lots of things, like wi-fi and Bluetooth.

In addition, with an IR system, the ambient light levels need to be low enough for the signal to register. An IR system doesn't work great outdoors in bright sunlight without bounce surfaces around, and you can lose range and reliability, which is why even manual-only radio triggering is so common for on-location shooters. In studio situations, though, IR triggering tends to work pretty well. And the bonuses of using a proprietary IR system are that a lot of the flash protocol is communicated between the camera and flash, so you have extra-fancy features like TTL communication, high-speed sync, group control, and remote power control. Some radio triggers can only communicate the sync signal, and nothing else. So IR vs. RF can be a tradeoff of features and reliability of signal.

Dumb Optical

In the case of the Di-866 you have both a transmitter and a receiver that can speak (if you got the Canon version), the Canon wireless IR protocol (which Nissin calls Wireless TTL). But you also have other "dumb" optical slave modes (SD and SF), which may be what's confusing you.

These systems are not proprietary and can be used with any flash signal. The sensor in this case simply fires the flash when it senses another flash burst. The SF mode fires on the first flash it sees, the FD mode fires on the second burst it sees. This second-burst mode is important if your on-camera flash is using TTL, because TTL fires out a "preburst" for metering, then the actual flash burst. If you use the SF mode, then your flash fires early. This type of simple optical trigger works in concert with studio strobes, or a manual flash burst from any system of camera (including point and shoots), so can be quite useful.

* BTW, the Canon wireless eTTL system is not, strictly speaking, IR. It's near-infrared but still uses visible light signals from the main flash head.

Canon's RF system

And, as a btw, Canon has a second wireless flash protocol, the RT system, that is RF-based, rather than optical. But the transmitters aren't built into any cameras, yet, so you need at least two units in the system. The 600EX-RT, and ST-E3-RT can be used as transmitters, the 600EX-RT and 430EX III-RT can be used as receivers. There are 3rd-party clones of the 600EX-RT out there, too (e.g., the Yongnuo YN600EX-RT).

Nikon also just announced their first RF-based flash unit (SB-5000), so it looks like we're moving away from the older optical-based systems to RF-based ones.

See also:

• Is there a Canon equivalent of the Nikon CLS?