I would like to better understand DTM implementation options for Target. The documentation covers only the most straightforward option when Target is implemented as a Tool. However, this has a few limitations:
1) The Target library will be loaded on every single page the DTM code is deployed on. This is not good when a single DTM web property covers more than one domain while Target should be implanted on a single site or even on a limited number of site sections.
2) It is not possible to deploy different library types (at.js, mbox.js) on different pages or site sections. For example, the mbox library may be required for compatibility with AEM, while at.js for the site sections working as SPA.
Because of mentioned above the following questions appear:
If yes, will it be always executed after MCID Service?
If yes, could you advise where to populate mbox parameters?
In addition to Mike's answers, I was able to speak gather a bit more information for your consideration.
We have a help article that goes over the DTM load order here. I spoke with one of our DTM engineers about this and your question and he indicated that loading Target via Page Load Rule is definately possible. One thing you may want to consider is moving all your sites to AT.js since Google Chrome plans to start displaying warnings in the console for document.write calls on 2G networks (documentation here).
Additionally, I can report that the MCID tool will be loaded prior to the top of page Page Load Rules.
I'm not 100% sure on the MCID question but would think the answer is yes.
To create the mboxes, we created other top of page rules and dropped in 3rd party sequential HTML (as the Target Tool isn't being used).
We don't have Target automatically create global mboxes, so things may differ a bit in that situation.