If anyone has hiked in the woods knows that noise generally will not travel very far. So the use of any whistle would imply that group members should be relatively close together.
It is of little importance what "standards" a particular group uses for their whistle signaling for communications, as long as everybody uses the systm correctly.
But that said I would be cautious on using a whistle too often in the outdoors.
This is the most important communication item. This affords a chance of self-rescue if one gets lost or separated from the group. A whistle may also be used to attract the attention of rescuers or passers-by. It requires very little effort and energy to blow, yet the sound carries great distances.
Children and immature adults must keep their lips off until an actual emergency. The whistle is a survival tool, not a toy.
All group members should be familiar with distress signaling. A universally-recognized distress code is three equal blasts on the whistle, to be repeated until others hear it and respond with two blasts of recognition. Make sure everyone knows that if possible, the person lost or in distress needs to stay in one place until reunited with the rest of the group.
Another universal distress code is the S.O.S (three short blasts, three long blasts, three short blasts).
Get a whistle that’s worth carrying.
Okay, there are whistles and there are whistles. Forget about kid’s toys and useless, feeble gimmicks. Get a good one, since a life could depend on it. Look for a whistle that’s rated for military, EMS, rescue, or law enforcement work (large, loud, durable, and easy-to-blow).
Put your whistle on a lanyard so you don’t lose it. - Wilderness Survival and Safety