The TARDIS holding ring was a section of the Doctor's TARDIS where the Doctor stored the accommodations of his companions after they had stopped travelling with him. It included the rooms of Susan, Ian, Barbara, Vicki, Steven, Dodo, Polly, Ben, Jamie, Victoria and on, and the final room was for Tamsin.
The Eighth Doctor brought Susan's TARDIS bedroom out of the holding ring as a Christmas gift for Susan. When Lucie Miller and Susan explored the holding cell, they found that the first and last rooms were next to each other. Before the space for Susan, there was a room expecting for her son Alex Campbell. After an accident with a Blitzen fish, the Eighth Doctor deleted all the rooms but one. (AUDIO: Relative Dimensions)
According to a different account, Susan's room was not immediately stored away, and it was later inhabited by Vicki, (PROSE: Scribbles in Chalk) Polly, Victoria, Sarah Jane, Leela, Nyssa, (PROSE: House) Tegan, (TV: Snakedance, Terminus) Peri (PROSE: House) and Sam. (PROSE: The Bodysnatchers)