Below is my summary (correct me if I'm wrong):
(1) A RSN is a network supporting RSNAs only. A BSS can be considered as a RSN if all of devices use CCMP/AES or TKIP/RC4 only.
(2) A pre-RSN is a network supporting pre-RSNAs only. A BSS can be considered as a pre-RSN if all of devices use static/dynamic WEP encryption and legacy authentication methods only.
(3) A TSN is a network supporting both RSNAs and pre-RSNAs. A BSS can be considered as a TSN if some devices use CCMP/AES or TKIP/RC4, while others use static/dynamic WEP encryption.
(4) WPA2-certified devices support the use of CCMP/AES, TKIP/RC4, and WEP/RC4. WPA-certified devices support the use of TKIP/RC4 and WEP/RC4. Neither WPA2-certified nor WPA-certified devices support the use of WEP/RC4 only.
Hope it does not sound confused... (again, correct me if I'm wrong.)
Based on the summary above, a 'WPA network' should belong to TSN since both RSN devices (using TKIP/RC4) and pre-RSN devices (using WEP/RC4) could coexist in a 'WPA network'.