There are two types of parts: outsourced and in-house.
An outsourced part is sourced from an outside supplier while an in-house part is manufactured in-house.
In-house parts and projects allow you to work with what's commonly called sub-assemblies. Essentially, an in-house part is used to track the part itself while a project is used to track, not only its revisions and bill of materials, but also every build of it.
Parts, in-house or outsourced, can all be assigned to BOM designators in projects.
It's important to notice the difference between stock and inventory. Stock is a specific and quantitative measurement. Usually, you say "it's out of stock" or you ask "how many do we have in stock?". The response is a number. On the other hand, when we talk about inventory, we might be interested in knowing where it is located, if it's on-order or reserved for some projects. In essence, inventory is a broader concept than stock, but both are useful.
The stock is displayed on the Parts table. This is the sum of your in-house and on-order inventory. This way, if you had low stock on some parts but you ordered more in the meantime (and didn't receive it yet), you won't see a warning or a misleading low value that would make you think you'd have to purchase more inventory.
Inventory is displayed on its own table, on the Parts details area.
From here you can get the answers for where and how your inventory is being used. On-order and reserved inventory contain a reference to the corresponding purchase order, project and project build.
Through the inventory table, you can manually adjust inventory by pressing the button. You can increase or decrease the quantity by entering a positive or negative number, respectively.
Inventory, projects and purchasing are all related, as inventory of parts is increased and decreased through finished project builds (the resulting in-house part get its inventory increased, while all the parts assigned in the BOM get their inventory decreased) and purchase orders, once received, increase the inventory of all the parts listed there.
You can also change the location of in-house inventory, or move a given quantity from one location to another. Bare in mind that, if you intend to move all the quantity available in one location to another one, it's normally better to just move that storage location instead (see Storage).
Part alternates are used to quickly replace parts in BOMs.
Similar parts share the same value and package but they aren't necessarily valid alternates of each other. Other specifications, such as tolerance and dielectric type might tell if it can be used as an alternate part or not. A listing of similar parts is provided by BOMIST but It's up to the user to define part alternates.