Strategy Builder Additional Data "Custom Series"?

So I was just curious what this is. In the strategy builder, under "Additional Data," there is a box called "Custom Series,"
to which you can add one of these:

Bool
Double
DateTime
Int
String

What are these, and how are they used? I looked through the NT8 Help Guide, and couldn't find anything about them. On that note, is there a book or a website or something I can read to help me understand NinjaTrader better, or do you think should I just keep digging around and stop to look something up or ask about it every time I have a question..?