What is the fundamental difference between combinational and mutational variability? What is the difference between mutational variability and modification?

The occurrence of mutational variability is based on changes in the genetic material: the structure of genes, the structure or number of chromosomes change. Combinational variability is due to the emergence of new combinations of parental genes in the offspring, while the gene structure, structure and number of chromosomes remain unchanged.
Mutations, unlike modifications:
– are inherited;
– do not develop gradually, but arise suddenly;
– do not form continuous series of variability and do not have a reaction rate;
– are non-directional (indefinite);
– manifest individually, and are not massive.