<processor> An Intel 486DX microprocessor
with its floating-point unit
All 486SX chips were fabricated with FPUs.
If testing showed that the CPU was OK but the FPU was defective, the FPU's power and bus connections were destroyed with a laser and the chip was sold cheaper as an SX, if the FPU worked it was sold as a DX.
[Was this true of all 486SX chips?]
Some systems, e.g. Aopen 486SX, allowed a DX to be plugged into an expansion socket.
A board jumper would disable the SX which was hard to remove because it was surface mounted.
Some SX chips only had a 16-bit wide external data bus
The DX has a pin to select the data bus width (16 or 32).
On the smaller SX, that line is hard-wired
to 16 inside the package.
This is similar to the 286 SX, which was a 16-bit processor with an 8-bit external data bus.
The Jargon File
claimed that the SX was deliberately disabled crippleware
The German computer magazine, "c't", made this same theory the basis of an April Fools Joke. They claimed that if one drilled a hole of a specified diameter through the right point on a SX chip, this would brake the circuit that disables the FPU.
Some people actually tried (and then bought themselves new processors).