What is the difference between settlement and subsidence?

I find the below explains best.

Settlement is like buying a new sofa and breaking it in so it gets nice and comfortable to sit in. Every sofa does it.
Settlement is usually a uniform sinking (more a kind of sighing) of the house. It happens when the house is newly built and it will stop.

Subsidence is like buying a sofa and a foot falls off.
Subsidence is a problem across part of the building which causes that part to start moving away from the rest. It needs rectifying or it will continue.

I have dealt with a property in Leek, Staffordshire Moorlands where the subsoil was different to the left side and was too weak to support the two storey house.
The whole side was re-built because the wall had gone too far.

Trees and clay are a bad combination, especially when the clay is highly shrinkable because it then has a propensity to expand when wet.
In normal circumstances the moisture level in the ground is fairly constant, but trees remove water from the subsoil. Sometimes this causes the clay to shrink, causing the foundations to sink.

Collapsing drains, culverts and, in rare cases, hidden mine shafts or bell mines can also remove the integrity of the supporting ground.
The presence of buried organic material, which then rots or breaks down, can also destabilise all or part of a foundation.

The solution?

Sometimes nothing. Sometimes the movement will stop and all that is necessary is a bit of re-pointing.

But if it’s serious then underpinning may be required. Holes are dug 1 - 1.5 metres long down to a good sub-soil that will support the weight of the building. This is done every so often along the affected section or wall beneath the existing foundation. Reinforcement bars are then inserted at 90 degrees into the side walls of these holes before they are filled in with concrete. The intervening sections are then dug out and similarly concreted, the reinforcement bars creating a homogenous foundation. It’s not rocket science but it is labour intensive and will cost around £1,000 for every metre.

One cheaper and less intrusive method is to sink a metal pile into the ground beside the foundation and build off that to the underside of the existing foundation.
Another is to “grout” beneath the foundation by injecting a cement solution that effectively turns the subsoil into an aggregate and consolidates the ground. This is not suitable for clay soild.