From SKYbrary Wiki
Stratiform, or layer clouds, are cloud formations that are not vertically developed. They are formed in relatively stable conditions, where lifted air will be restricted, and instead be spread out horizontally. The cloud base may be quite low, if not actually touching the ground (fog). Cloud top is determined by the lifting/mixing of the air and it's water vapour content. The shape or structure of the cloud top, with the exception of significant features is due to atmospheric conditions (warming, cooling, wind, etc) at the top and not an indication of wind at the surface.
Layer Clouds are found in high level clouds (Cirrus, Cirrocumulus, Cirrostratus), mid level clouds (Altostratus, Altocumulus), and low level clouds (Stratocumulus, Stratus). Nimbostratus, another layer cloud, may extend from low level to mid level. Fog is stratus cloud which is in contact with the ground.
A stationary line of stratiform cloud may indicate a convergence of airflow due to topography or water temperature over water surfaces.
A moving line of layer clouds may indicate a change in wind speed and/or direction. The directional change is generally a veer, as the line is similar to a small scale trough. When there is no wind, an advancing line of stratocumulus may result in wind from the left of the direction of the line movement. If the line is composed of stratus or fog, less wind on passage can be expected due to the effect of friction and nature of fog/stratus.
Stratiform cloud may form during a warm front passage due to the steady, continuous rain falling from nimbostratus cloud, adding substantial amounts of humidity to the air. The cloud may form ahead of the front, in the cold air mass, or behind the warm front in the warm air mass.