Overview
The value approach to trading seeks to formulate a theoretical value for an asset, that is, a fair price that would paid by an agent using rational decision making. Then, the strategy will purchase the assets that appear under valued and sell the assets that are over valued.
The models used to determine a theoretical value is dependent on the asset in question. For equities, there is an abundance of data, most of which comes from the finanical statements of the company. For other assets, such as crypto currencies, the amount of data from which to draw to determine value is marketably less.
Theoretical Value
The beginnings of most models of value is usually present value. That is, most instruments are valuable because they lead to a cash flow or a stream of flows in the future.
Some instruments, such as equities and bonds, have periodic cash flows that are distributed to the holders (dividends for equities, interest payments for bonds) which can then be fit within the present value framework.
Of course, many of the cash flows are not deterministic. That is, they are not necessarily known in advance and may be assumed to random. The present value framework can still be applied, but may require some sophistication in assigning a discount rate. (see stochastic discounting)
There are limitations to applying the present value methodology, and this may require asset specific frameworks or heuristics.