Scratch space is an additional directory where you can store 'disposable' data without it counting towards your main disk quota.
There is one very important proviso regarding scratch space: Data stored there is at your own risk - it is not backed up at all, not kept on redundant storage, and not guaranteed to survive disk failures, machine crashes or anything else.
When you may require scratch space
Scratch space is particularly useful where you are generating large amounts of intermediate data that:
Can be replaced fairly easily in case of data loss
Won't fit in your home directory quota
Requires high-speed or intensive access for heavy computation
Would generate unreasonable network bandwidth if kept on a file server
Requesting scratch space
To request access to scratch space, speak to your supervisor regarding your requirements. If there is space available on an available machine (such as one of the research group clusters), they can pass on your request to SS, who can grant you access.
Drawbacks of using scratch space
Scratch space, by definition, is temporary or intermediate storage and is provided with certain conditions:
It is local to the machine or server it is created on
Available space is dependent on the resources of the machine hosting it
Disk space is managed by the supervising academic - you may be asked to reduce your usage to accommodate other users