什么事non-EPS服务

如题。
看协议，detach的服务有EPS services和non-EPS services。

这个non-EPS services是什么？是指短信这样的业务吗？

再问个问题，关于LTE detach类型，看了一下协议再搜了搜网上的说法
 EPS detach  --- 这个比较好理解，就是非关机去附着，不做业务。


IMSI detach ---  有些人说是关机去附着，有些人说是正常去附着的一种。到底是关机还是正常去附着呢？如果是正常去附着，又是哪种去附着呢？

combined EPS/IMSI detach --- 这个又怎么理解呢？

根据3GPP 24301中的介绍
attach的目的：
The attach procedure is used to attach to an EPC for packet services in EPS.
The attach procedure is used for three purposes:
-    by a UE in PS mode of operation to attach for EPS services only;
-    by a UE in CS/PS mode 1 or CS/PS mode 2 of operation to attach for both EPS and non-EPS services; or
-    to attach for emergency bearer services.

detach 的目的：
by the UE to detach for EPS services only;
-    by the UE to disconnect from the last PDN it is connected to;
-    by the UE in CS/PS mode 1 or CS/PS mode 2 of operation to detach for both EPS services and non-EPS services or for non-EPS services only via a combined detach procedure;
-    by the network to inform the UE that it is detached for EPS services or non-EPS services or both;
-    by the network to disconnect the UE from the last PDN to which it is connected; and
-    by the network to inform the UE to re-attach to the network and re-establish all PDN connections.
The detach procedure also applies to the UE which is IMSI attached for "SMS only".