I have developed in Visual Studio solution that define custom field type for SPList. Everything works fine. After deploy I was able to add my custom column into SPList. SP has created field ID (Guid).
I have noticed that some SP fields has always the same guid, for example blog body: 7662cd2c-f069-4dba-9e35-082cf976e170. It's easy to find column for SPListItem id you know ID.
Question: Can I define my custom Guid that will be used by SP to set field id in data base?
The reason the Guid is same for the 'blog body' is because its a site column. Site column is a resuable column definition (based on a type), that can be reused in multiple lists.
If you create a column in a list, that column is local to that list and the Guid will be different from all other columns. If you would like to reuse a certain column (usualy a part of content type), you would create a site column and add the site column to the list/library. In this case no mater where you use the site column in the target site collection, the Guid will be the same.
Note that you cannot set a guid, when creating a site column using UI. But you can control the guid if you are deploying it via code. Look at this for more info.