3GPP TS 24.301

6.5.1.2 UE requested PDN connectivity procedure initiation

When the PDN CONNECTIVITY REQUEST message is sent together with an ATTACH REQUEST message, the UE shall not start timer T3482 and shall not include the APN.
ATTACH REQUEST消息中携带的PDN CONNECTIVITY REQUEST不应携带APN。
NOTE 1: If the UE needs to provide protocol configuration options which require ciphering or provide an APN, or both, during the attach procedure, the ESM information transfer flag is included in the PDN CONNECTIVITY REQUEST. The MME then at a later stage in the PDN connectivity procedure initiates the ESM information request procedure in which the UE can provide the MME with protocol configuration options or APN or both.
在Attach过程，如果UE需要提供PCO和APN给网络，则ESM information transfer flag需要置为1。
在Attach过程，如果ESM information transfert flag为0，即网络会下发PCO与APN给UE。
参考文档：手机通信问题集锦/典型案例分析：PCO要放在ESM information response中？

8.3 EPS session management messages

8.3.20 PDN connectivity request