What is Fortinet IPSec VPN Log(tunnel Down) condition?

Hi

We are using the 3 ipsec VPN(AWS to Fortigate 500D)

When IPsec VPN Failure one is only occured the log(phase 2 Down)

the other IPsec VPN is occured the three logs (DPD failure, tunnel Down, phase 2 Down)

Can I get the detail conditon about tunnel Down, DPD failure Log?

And is it possible to occur phase 2 Down Log without tunnel Down?

Device : Fortigate 500D

Version : FortiOS 6.2.3