Types of Funds

I understand there are different types of Defense Department R&D funding, e.g., 6.2, 6.3, 6.4. When I asked someone for more information, it was explained to me that different types of funds had different codes depending on their intended purpose. For example, Basic Research is 6.1, Applied Research 6.2, Advanced Technology Development 6.3, etc. However, that was the extent of the knowledge.

I'm wondering if someone can point me to something that explains the nature of these classifications/codes. I'm assuming these numbers refer to something specific and 6.1 was not just grabbed out of thin air.

What actually establishes these different types of funds? Are they budget apportionments, allotments, something else? Also is there something that expands on/defines when an effort would properly fall into one category or another? For example, what would you consult to determine whether 6.4 funds (as opposed to 6.3 or 6.5) could be used for a proposed contract?

Thanks.