There are 5 main statuses in Zendesk:

Let go through them and see what each of them does.

New:

New status is automatically assigned by the system the moment ticket is created. Agents cannot manually set New status.

New status - counts SLA.

New status - changes to open upon ticket assignation to age.

New status - changes to any other status upon manual selection (except for solved).

There is no real difference between New and Open statuses in terms of technical functionality.

Open:

Open status is used for all issues that are not resolved and active ticket resolution is in process.

Open status - counts SLA.

Open status can be changed to Pending, Solved, On-hold.

Pending:

Pending status is used when we need additional information from the customer in order to continue with the ticket resolution. Think about this status as "Waiting for additional information from customer".

Pending status - stops SLA.

Pending status - changes to Open on customer reply.

Pending status - can be set to Solved or Open manually.

Pending status - sends notification to client everytime it is being set, so use carefully (don't set it without public reply or client will get confused).

Pending status - changes to Closed automatically in 8 business days if client sends no reply.

Solved:

Solved status is used when we have the resolution ready and are ready to share it with the customer. Think about this status as "Issue resolved".

Solved status - stops SLA.

Solved status - changed to Open on customer reply.

Solved status - can be set to Pending or Open manually.

Solved status - sends notification to client everytime it is being set, so use carefully (don't set it without public reply or client will get confused).

Solved status - changes to Closed automatically in 3 business days if client sends no reply.

On-hold:

On-hold status is used when we have to wait for information from 3rd party.

On-hold status - counts SLA.

On-hold status - can be set to Open, Pending, Solved manually.

On-hold status - sends notification to client everytime it is being set, so use carefully (don't set it without public reply or client will get confused).

On-hold status - stays for indefinite amount of time until manually set to a different status.

Summary:

New, Open, On-hold - SLA counted.

Pending, Solved - SLA stopped.

Notifications to customers are being sent everytime you set status to Pending, Solved, On-hold.