Hi Kumar,
There is no specific check box which will say sub contracting in service activity. But there is a check box "Service exist" which will be populated automatically.
In standard SAP, If the control key is PS04 then it is service activity.
Enter the relevant purchasing info and press enter. This will take you to another screen where you can enter the service specifications.
Based on the config, you can generate PR with services and can be converted to service PO.
Please let me know if you need more help on this.
Thanks
Regards
Srinivasan Desingh