OCR Advanced GCE in Chemistry/Periodic Table: Transition elements/I don't get this?

At AS level we learnt the following about writing electronic configurations:
 * We write the configuration showing the numbers of electrons in each sub-shell
 * An s sub-shell contains 2 electrons, a p 6 and a d 10
 * Each successive shell contains one more sub-shell (e.g. shell 1 only has an s sub-shell, shell 2 has an s sub-shell and a p sub-shell)
 * The 4s sub-shell fills and empties before the 3d subshell (very important in this topic)
 * If the configuration is really long we can use a core and then add the extra electrons on the end e.g. using an argon core for scandium - [Ar] 3d1 4s2