Kindly refer to the notification No 8/2005 ST dated 1/3/2005. Activity of a Job worker may or may not amount to maunfacture. It is not proper to say that if a activity is a manufacure then the person carrying on such activity is not a job worker. If the job worker manufactures excisable goods and the same are used as input by his client, then the job worker has two options. Either he can pay excise duty and enable the client to claim credit of the same or the job worker can claim the benefit of CE notification 214/86 - CE.
Therefore, if the job worker has performed any activity on the goods supplied by the client and the same goods are used by the client for manufacture of excisable goods, ST is not applicable.