Hi Pranav,
According to the salesforce documentation :
Assignment rules automate lead generation and support processes for an organization.
There are types of a assignment rules:
Lead Assignment Rule :-Specify how leads are assigned to users or queues as they are created manually, captured from the web, or imported via the Data Import Wizard.
Case Assignment Rule :- Determine how cases are assigned to users or put into queues as they are created manually.
In Your Org there can we one Assignement rule active on particular Sobject