Raster Analysis tasks that have the mask property in their context parameter will only consider those cells that fall within the analysis mask in the operation for the output raster.
There is no mask specified by default. The mask can be a raster or a feature dataset. If the analysis mask is a raster, all cells that have a value will be considered to define the mask. Cells in a mask raster that are NoData will be considered to be outside the mask and will be NoData in the output. If the analysis mask is a feature dataset, it will internally be converted to a raster on execution. For this reason, you should ensure that the Cell Size and Snap Raster are set appropriately for your analysis.