The Roman Archive of the Society of Jesus is the archive of the central government for the worldwide Society of Jesus. In this sense, it doesn’t collect local archives but mainly only the documents of the General Curia (central government of the Society).
Netherless, ARSI is a valuable source of information for local history, but in the way this was linked to the central governament (for example relations sent to the Father General and so on)
Execept few cases, ARSI doesn’t preserve personal fond of famous Jesuits, but only the ones of the Gerenal Fathers of the New Society.
Even the letters preserved are the corrispondence beetwen Jesuits and Fathers General. Instead, for example, the corrispondece between a Jesuit and a Father Provincial should be seached in the archives of the provinces, not here.
If you are aiming to consult the documentations held by ARSI, please write a request of reservation.
For a general presentation of the main fonds that you can find in ARSI see this article of Fr. Danieluk SJ.
To understand some of the type of documents cited in that article, see also the Gramatowski glossary in the section general repertories.
If you are a research from Nord America, please note that the University of St. Louis has a copy of part of documentations of ARSI in microfilm