Types of Tickets in Production

What are the types of ticket and its importance?
This depends on the SLA. It can be like:
  1. Critical.
  2. Urgent.
  3. High.
  4. Medium
  5. Low.
The response times and resolution times again are defined in the SLA based on the clients requirement and the charges.
This is probably from the viewpoint of Criticality of the problem faced by the client as defined by SAP.
  1)      First Level Ticketing:
Not severe problem. Routine errors. Mostly handled by Service desk arrangement of the company (if have one).
Eg: a) Say Credit limit block in working on certain documents?
      b) Pricing Condition Record not found even though conditions are maintained?
      c) Unable to print a delivery document or Packing list?
PS: In the 4th phase of ASAP Implementation Methodology( i.e Final Preparations for GO-LIVE) SAP has clearly specified that a Service desk needs to be arranged for any sort of Implementation for better handling of Production errors.
Service desk lies with in the client.
2)      Second Level Ticketing:
Some sort of serious problems. Those Could not be solved by Service Desk. Should be referred to the Service Company (or may be company as prescribed in SLA).
Eg: a) Credit Exposure (especially open values) doesn't update perfectly to KNKK Table.
      b) Inter company Billing is taking a wrong value of the Bill.
      c) Need a new order type to handle reservation process
      d) New product has been added to our selling range. Need to include this into SAP. (Material Masters, Division attachements, Stock Handling etc.)
3)      Third Level Ticketing:
Problems could not be solved by both of the above, are referred to Online Service Support (OSS) of SAP Itself. SAP tries to solve the Problem, sometimes by providing the perfect OSS Notes, fits to the error and rarely SAP logs into our Servers (via remote log-on)for post mortem the problem. (The Medical check-up client, connections, Login id and Passwords stuff are to be provided to SAP whenever they need or at the time of opening OSS Message.)
There are lots of OSS Notes on each issue, SAP Top Notes and Notes explaining about the process of raising a OSS Message.
Sometimes SAP Charges to the client / Service company depending on the Agreement made at the time of buying License from SAP.
Eg: 1) Business Transation for the Currency 'EUR' is not possible. Check OSS Note  - This comes at the time of making Billing.
      2) Transaction MMPI- Periods cannot be opened – See OSS Note.
      There are many other examples on the issue.
4)      Fourth Level Ticketing:
Where rarely, problems reach this level.
Those problem needs may be re-engineering of the business process due to change in the Business strategy. Upgradation to new Version.  More or less this leads to extinction of the SAP Implementation.