In theoretical sampling, participants are selected in order to inform the researcher's developing understanding of the area of investigation.
It is often used in the grounded theory research in order to develop a theory throughout the research process itself.
The idea is that the researcher collects data from any individual or any group of people who can provide the appropriate and relevant data for the generation of his or her theory.
Note - this is the same as theoretical sampling that you have looked at previously in this web program.
quantitative - samples qualitative - samples selecting participants