The idea here is to keep some degree of consistency as you go from sprint to sprint by setting up definitions for each Story Point value that the Scrum Team understands and can work to. The definition should be general enough to work with just about any kind of User Story. The alternative is to have different definitions for different types of work and then map the 2's, for example, for each of these definitions to each other.

To set this up, work with the team to create definitions that everyone can agree on. For example, the team might have found that a small GUI change with no new business object is something they consider to be a 3, whereas something with a large GUI change might start of by being an 8, with 13 being used if it also means a new business object is required as well.

Teams have found this helps stabilize the Actual Velocity, helps show when the Scrum Team is improving, or not, and generally provides more confidence for the estimates. It works in conjunction with Establish a Keystone User Story. To help he team figure out things like complexity you could look to Establish a "Things That Matter" Matrix