This read only array holds all the ids of every active instance within the room. This means that if you have used any of the Instance Deactivate functions those instances that have been deactivated will not be included in this array (if you have used a value from this array previously, it will now return the keyword noone).
Real (instance ID value)
for (var i = 0; i < instance_count; i ++;)
with (instance_id[i]) speed += 0.1;
The above code will loop through all instances within the room and add 0.1 to their speed.