"Adult learning" is a general term that could entail many specific educational models such as adult "continuing education" programs offered by universities and community colleges, as well as trade or technical schools.
In the context of Human Resources Development, adult learning could be seen as a subcategory within the field of HRD. Contemporary human resources development encapsulates various subfields such as education and training of employees.
Since today's economy and job market requires education attainment as a parameter or qualification for employment, it would be easy to assume that adult learning should be part of human resources development.
Adult learning is already a subfield within the broader education field and therefore it arguably does not need to be a discipline of its own. However, adult learning, along with the psychology subfields that analyze learning mechanisms of adults, should be studied by HRD students.
It is important for HRD students to understand the various levels of education that employees attain and that employers or organizations seek. Learning this subfield will also allow HRD students to evaluate the skills of the employees.