DCS or, as they called it, DPL, was developed by Motorola back in the 1970s, Originally, 
there were 777 codes plus 777 inverted codes. However, that just did not "work out" and 
there were a lot of false signals which would "open" the squelch even though the correct 
code was not sent. The result was that the practical number of DCS / DPL codes were 
reduced considerably.

CTCSS is a LOT easier to apply to the transmitted signal. The audio is normally applied 
after the frequency limiting microphone circuitry, usually at the wiper of the deviation 
("modulation") control. As such, CTCSS can be used with PM transmitters as well as true 
FM transmitters. DCS / DPL is normally applied to the r.f. section of the transmitter 
(rather than through the audio chain) often right at the original oscillator.

Motorola had in mind employing a system which would not allow any equipment manufactured 
by any other company to be used in that system. However, a Canadian company, Ferritronics, 
developed a DCS modulation scheme that worked perfectly with Motorola DPL without 
violating any patents. Those boards could be incorporated into any transmitter and 
receiver and thus allow full compatibility from any radio with the Motorola system. 
Ferritronics also incorporated a feature, enabled by cutting a jumper on the adapter 
board, which prohibited radios equipped with Motorola DPL from working with the Ferritronics 
system. As such, systems could be sold by other manufacturers that would not allow Motorola 
to sell radios into those systems although any other manufacturer, using the Ferritronics 
boards, could produce equipment capable of operating within those systems.

Of course, desired transmissions would have to have either the correct CTCSS tone or 
correct DCS coding to be received. However, any receiver can receive the encoded 
signals just by operating in the "standard squelch" mode.