They offer DGD coin which is like shares except you must vote to get your share of the profits; plus have it locked up in a ERC-20 wallet 3 months prior to voting for changes to the software it represents which is the digix marketplace. This is known as their DAO.
The separate token DGX is what is represented to the gold bullion in a ration of 1 DGX = 1 gram of gold. A minimum redeeming amount is 100g of gold or 100 DGX tokens. There are also a fee charge to your DGX coin if you store for more than 24 hours, it's a fractional amount that goes into the voting pool to pay out DGD holders who actually vote.
Also, they have a reserved amount since the funding of the project set aside to pay out DGD holders each quarter for voting.