well, that's an interesting question!
You have to distinguish between the read order of profiles and the order of sources how profile parameters are replaced:
First of all - the Start-Profile was a bad design in older SAP releases, because it was only used by the startup framework (sapstartsrv.exe - startsap). In the current SAP releases there is only one profile which is used for the instance configuration and the startup framework: The instance profile.
The files are read in following Sequence:
1. Instance Profile (name: <SAPSID>_<InstancenName><no#>_<hostname>
2. Default.pfl
During the profile parameter value resolution the order is as documented in report RSPARAM:
1. hard coded value
2. Default.pfl
3. Instance Profile
What does the resolution order mean:
A hard coded value of a profile parameter (almost all profile parameters do have hard coded (default) values) is overwritten by the value - if the parameter is present in the default.pfl.
The value of a default.pfl parameter is overwritten by it's instance profile parameter value.
If you have multiple instances in one system you are placing a parameter, whos
value is identical for all instances into the default.pfl. A parameter whos value is only valid for a specific instance is placed into the instance (specific) profile.
In theory it is also possible to use one instance profile for several identical configured instances. The profile parameter system will work fine for this (but not SWPM / SAPInst).
Kind regards
Peter