E-UTRAN is described in detail in TS 36.300
with additional functions listed in TS 23.401
Trusted and Untrusted Non-3GPP Access Networks are IP access networks that use access technology whose specification is out of the scope of 3GPP.
Whether a Non-3GPP IP access network is Trusted or Untrusted is not a characteristic of the access network.
In non-roaming scenario it is the HPLMN's operator decision if a Non-3GPP IP access network is used as Trusted or Untrusted Non-3GPP Access Network.
In roaming scenario, the HSS/3GPP AAA Server in HPLMN makes the final decision of whether a Non-3GPP IP access network is used as Trusted or Untrusted non-3GPP Access Network. The HSS/3GPP AAA Server may take the VPLMN's policy and capability returned from the 3GPP AAA Proxy or roaming agreement into account.
For supporting multiple PDNs, the same trust relationship shall apply to all the PDNs the UE connects to from a certain Non-3GPP Access Network, i.e. it shall not be possible to access one PDN using the non-3GPP access network as Trusted, while access to another PDN using the same non-3GPP access network as Untrusted.