The ATPL Sidewise Sideways ROM board provides ROM and EPROM expansion capacity internally to the BBC Micro, allowing the machine to access a full complement of 15 installed ROMs. ROM bank 15 is split into two banks of 8KB of RAM which allows for a ROM image to be loaded into the ROM bank or for the extra Sideways RAM to be used under certain circumstances by expanded games that took advantage of the memory if fitted.
Fitting the board can be a solderless affair however, if you're handy with a Soldering Iron, the job can be made much more professional by changing a couple of sockets and altering the power connections on the motherboard slightly.
The standard instructions for fitting the ATPL board simply cover removing the two IC's from IC73 and IC51, connecting the two flying leads to the jumper header J21, bending the two original spade connectors on the motherboard to make some room and pressing home the board into the sockets. of IC73 and IC51.
To make the job a little more professional and reliable in the long term it's worth doing the following whilst the BBC Micro is in bits.