Sorry for the length of this.
The game has Units (single ships) and Groups (one or more ships) in the SCR layer, and random groups in the RND layer, which can be singles or multiples of ships. Single Missions can have both, but you have to be careful how you "Save". The basic structure in all is the same, so a Unit has
[Unit 1] - header, numbered incrementally
Name=BR DD V&W#3 - yours, or the game's name - MUST be unique
Class=DDV&W - ship Class
Type=4 - 4=DD Type
Origin=British - country of origin
Side=0 - 0=Neutral, but the "date" can change that
Commander=0 - Game=0 Human=1 controls the ship / plane
CargoExt=-1 - number designating type of cargo -1 is NONE
CargoInt=-1 - same as above
CfgDate=19380101 - controls the eqp that shows on a ship, like guns
DeleteOnLastWaypoint=false - or true. This ship "loops"
DockedShip=false - or true. Will it stay in one place?
GameEntryDate=19390801 - when it shows in game
GameEntryTime=0 - and the time it does
GameExitDate=19400411 - the date it exits the game unless terminated by other means (sinking)
GameExitTime=0 - and the time
EvolveFromEntryDate=false - this determines if its location updates when the player enters the game
Long=-694700.000000 - location in decimal
Lat=6242900.000000 - same as above
Height=0.000000 - for submarines & planes only
Heading=273.739990 - where is it pointed?
Speed=12.000000 - how fast initially
CrewRating=1 - competency, 0-4
DelayMin=0 - for the reporting
ReportPosMin=-1 - any negative number means "No"
ReportPosProbability=100 - used if above is positive, then an icon will show on the NavMap
RandStartRadius=0.000000 - how close to lat/long will it spawn?
NextWP=0 - counts from zero, so first waypoint is used here
There is also a "Commander" in the Group set-ups, but that points to the "command" ship, and starts counting at zero. While the layout of a group or RndGroup are slightly different, all of that basic information is in there. The regular "Group" will have a "naming" segment, then the first ship, all of the waypoints, and then the rest of the ships. Concerning what you are asking, the
EvolveFromEntryDate influences whether the scripted [Unit] ship is somewhere along its routing, appropriate to when you enter the game (set to "true"), or if it will start when you start, which is the case with the "false" setting above.
RndGroups do not have an "EvolveFromEntryDate" setting. A RndGroup has the Name Grouping, then the ships or planes, then the Waypoints. Besides the info being re-arranged in the RndGroup, the "delay" in the Random Groups "spawning" is found in
...
Height=0.000000
DelayMin=60
DelayMinInterv=2220
SpawnProbability=90
RandStartRadius=20.000000
...
"Orange" lines. The "DelayMin" tells the game how long to count before it begins the GameEntry counting... whuh? In some cases, that figure will default to 1440, or 24 hours... In this case, it is one hour, or 60 Minutes. You can set that to zero (0) if desired. I usually set it to one (1), and it is usually left alone by the ME. The "DelayMinInterv" is the amount of time to count before attempting another "random" spawn. In this case, that would be 37 hours, or a bit over a day and a half. That would make it be later each day... The "SpawnProbability" is almost self-explanatory, so in this case, 9 out of 10 times, after 37 hours has passed, the game will spawn this group... The game tracks the RndGroups by their "CurrentInstanceID" setting, which you should not touch.
When dealing with either kind of grouping, make certain that you use enough appropriate spacing between ships in the
ColumnsNo=1
Spacing=500
The game sets the minimum for ships at 500, but 1000m is necessary for the big ships. Airplanes minimum is 300m, but should be at least 700... If you see a group not "convoying" very well, their spacing is too small - they do not maneuver very well... Note also that all of the campaign files do load during SingleMissions, unless the mod makes provisions for two installs, so some of what you see in a SingleMission might actually be part of the game's campaign files.
If you want something to run one time, then you use the SCR layer. If you want it to run multiple times, then you use the RND layer. The LND layer is for NavalBases, AirBases, and other shore emplacements.
I probably forgot something...