what is meant by sub-assembly?

Answer 1

Sub-assemblies are portions of the finished product that are put together ahead of time -- often by someone else.

Picture a desktop computer. Many people "build their own computer". In reality all they do is attach sub-assemblies. They purchase a motherboard, video card, hard drive, memory modules, a CPU and a case.

Here are some of the sub-assemblies they took for granted:

Motherboard have dozens (hundreds?) of components on the circuit board.

the hard drive is a complex system of magnetic media, moveable heads, and a host of other items.

The case often consists of the physical enclosure plus a power supply and the wiring harness.

The video card and memory modules are sub-assemblies of components already soldered to circuitboards that will ultimately plug into the motherboard.

The CPU is perhaps the closest to a single component, however even it is made up of millions of transistors on a silicon wafer. Another sub-assembly.

Another example:
Cars are now produced in "assembly plants". Few are referred to as factories since nearly everything is pre-made off-site.

Answer 2

A sub-assembly is a part of the main assembly.

Then the sub assembly will have to be put together, assembled, and then that entire assembly gets put togther with the main assembly.

Example, your car is the main assembly, the drive train would be a sub assembly.

Answer 3

A sub-assembly is part of a device that can be assembled (and often tested) separately and installed as a unit.