The SRA Series
The SRA Series is the ISO paper size standard that is used by printers to produce the finished A Series of paper sizes.
If you need a finished job of A3, a printer uses a sheet size of SRA3 or alternatively an SRA2 sheet, out of which he would get 2 A3’s.
This slightly larger paper size allows room for any bleed on the job as well as colour bars that printers use to ensure that the finished printed sheets are the correct colour. This slightly larger sheet size also allows for an edge that the print press can grip onto as the sheets of paper pass through the it.
It is very unlikely that you will need to know these paper sizes unless you are involved on the printing side of the industry.
The RA Series
The RA Series is another alternative ISO paper size, again designed to produce the finished sheet sizes of the A Series but for jobs that do not have any bleed on them. It was predominately used for printing single colour stationery without bleed but today his series is not widely used.
The B Series
The B Series is a less well known ISO standard for paper sizes that is used by printers to print jobs that do not quite fit onto a standard SRA size. The most common use of this paper size if for folders with pockets that do not economically fit onto an SRA sheet size. Again this isn’t a series that is often sited but it also allows you to produce over size A4 documents in an economical fashion that can make the finished job look and feel a little different. The B series is used in specific industries that require slightly larger finished jobs.