The phrase "they/you know who they/you are" is typically used to address members of a subset of your audience without specifying their names.
We often use this phrase in an acknowledgement, or dedication, like you might find at the beginning or end of a book. An example would be:
This book is dedicated to all the people who supported me as I was writing it. They know who they are.
In other words, those people who supported me know that they are included in the group "all the people who supported me", so I'm not going to list each of their names.
Another common use - and one that perhaps makes more sense - is when you're admonishing a group of people in public, but want to spare them the embarrassment of being singled out. For example:
Some people have not yet completed the required training - they know who they are (because we sent them a reminder email) - and we're working with them to get the training finished as soon as possible.
Similarly, you might use this phrase when giving a public address because you want the general audience not to worry that you might be talking about them:
For those of you who forgot to sign up for a shift - and you know who you are - please email Alice by the end of the day.
In the quoted example, the author intends to say something like this:
The next section is not for everyone. It's only for people who need to know extra information about [whatever the next section is about]. The people who need to know this information are electrical engineers and systems designers who create computer motherboards and if you were a member of this group, you would know you were a member. So if you're not sure if this next section is for you, it's not.