A field with a NULL value is a field with no value.
If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value.
Note: It is very important to understand that a NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation!
Service accounts are all about security and access. So, for example, SQL Agent runs as a service and it can be configured (should be) to run under a service account. Let’s assume that you use Agent to run backups. Let’s also assume you backup to a shared file location on your network that’s not local to the machine you’re running SQL Agent on. You’ll need to ensure that the account configured for SQL Agent has access to that shared file location. While this may seem like work, what it in fact is doing is following the method of least access. That service account has to have access to that share, but it doesn’t need access to other file locations on your system, so you only give it what it needs and nothing more. Same thing applies to the other services and service accounts.