Reservations are always tricky systems to implemented both in a inventory system and in the physical management of the goods. What do you do if you have limited stock and class b customer orders first and you have reserved inventory but then you get an order for a class A customer. Do you take stock away from the first customer for example. These are always interesting policy and procedure issues to have to deal with. In AX you can manually reserve items at the sales order or you can automatically reserve.
Here is a quick overview
AX2012 R3 CU9.
As you can see in the video you can turn on automatic reservation either at the item model group level or for all orders then you can enable it on the accounts receivable parameters.
In this example we are also just showing the reservation process at the sales order. If you are using the advanced warehouse then you could delay reserving orders until they are released to warehouse. You can get a overview from a previous post.