Configuration profiles could be installed on iOS (through iPCU, MDM, web an so on).
Each configuration profile may have multiple payloads. Based on the the documentation, PayloadContent is an array of payloads.
I wonder, whether the order of payloads installation is predefined or undefined.
On one hand, if it's array, my assumption would be that they are installed in predefined order. On other hand, I didn't find any information on this subject.
Generally speaking, @tc who wrote a comment is right. It's better to not rely on undocumented functionality.
However, I experimented with this and found that payloads are installed in order in which they are specified in the array.
Probably, the only exception would be identity payload which are referenced by WiFi, VPN or MDM. However, I didn't try them.