In Infinite Flight, to simplify things, a straight out departure simply means aircraft should maintain runway heading until no conflict exists. When in doubt, fly straight out.
3.2.8 — When “Straight Out Dept.” is being broadcast in the current ATIS (see 4.2.1 below), the use of “fly runway heading until at or above XXXXft” should be used with every take-off command sent as a final reminder to the departing aircraft; the options available include 1000ft, 3000ft and 5000ft AAL (rounded down to the nearest 500ft). Aircraft climb rates can vary significantly, thus; although aircraft have been instructed to maintain runway heading until a specific altitude, with “Straight Out Dept.” selected in the ATIS, aircraft cannot deviate from runway heading until the departing aircraft is clear of any conflict.
Tower should instruct you to maintain runway heading until you reach a specific altitude, which will be specified in the takeoff clearance. Even if you haven’t received this extra reminder, you should still maintain runway heading until you are clear of any conflicts and depart straight out.
3.2.9 — SIDs often involve turns away from the airport shortly after departure; due to this, if “Straight Out Dept.” is being utilized, Controllers should note that this may prevent pilots from flying the selected SID (see 4.2.1 and 6.4.3 below).
As noted in 6.4.3:
6.4.3 – […] Controllers should let aircraft continue on a SID to the maximum extent possible after departure (therefore vectors and/or altitude assignments are not required on initial contact) unless required for sequencing or traffic avoidance. Due to this, Controllers should limit the use of “Straight Out Dept.” to accommodate SIDs which often involve turns away from the airport shortly after departure (see 3.2.9 and 4.2.1 above).