Immediately doesn’t have any effect on timing rules, but is there to help place emphasis. Looking at the timing chart, both abilities trigger at the end of movement so they owner would get to choose their order or resolution.
Note that the original question, as asked in Discord, wasn’t whether the active player could choose the order of resolution, but specifically whether Vayl could cast Slipstream, advance, end movement more than 2" away from a warbeast with Tether on it, advance the Tethered warbeast to within 2" of Vayl, and then place the warbeast using Slipstream.
When Vayl ends her movement , the distance between her and the Tethered model is already determined, even if the Timing Appendix allows you to resolve Tether first (something I’m not concinced of).
This is similar to the old Counter Charge & Dark Sentinel issue, wherein an enemy model ends within 5" of Vayl but >6" of the Counter Charge model (e.g. Raek). Even if you use Dark Sentinel to move the Raek closer, the distance between the enemy model and the Raek when the enemy lodel ended its movement was already determined.
Could this be added to the rulebook somewhere? There have been countless questions, past and present, regarding “Immediately”, “After,” etc. A quick section in the timing area would be extremely helpful as it would clearly show how the dev team uses words.
I’ve always thought that the movement timing chart would benefit from splitting it into a few more steps for these interactions (as discussed in the thread about the timing of simultaenous placement of troopers).
I really, really appreciate making the timing chart a separate file in the App, by the way. It’s probably the most important part of the rules when resolving rules questions, so having it easily accessible is a great benefit.