If you want to apply a discount to a job in particular:
To add a discount to a job, you have to click on Edit in the Billing panel of this job and select the option to add a discount. You will then be able to define a percentage for the discount.
This discount will be automatically taken into account when you invoice the job.
If you want to apply a general discount to all the jobs in a document:
When you're in the invoice or quote, you can click on 'Add discount' below the subtotal to add a discount (in %) that will apply to all the jobs in the document.
To remove a discount you applied to a document, click on the discount below the subtotal and unselect the 'Add discount to the invoice' option. Then, click on Save. You'll see that the discount will be removed.
If you want to apply a discount to a certain line in your document:
To apply a discount to a certain job in an invoice or quote, you have to go back to the job, add the discount there (see above), and then go back to the invoice or quote and click on the three dots at the end of the line concerned and then on 'Sync with job data'. This will update the document based on the changes you made at the job level.
If you're using LSP.expert V1
To define a discount for a job, you have to click on the estimate of the job (on the left) and edit the Discount field by clicking on the little pencil.
This discount will be taken into account when creating the invoice for the job.
If you want to apply a general discount on all jobs in the document:
There is a Discount field in every invoice/quote. That's where you can define the discount (in %) you want to grant to your client.
If you want to apply a specific discount on certain lines in your document:
There is a Discount column where you can define a specific discount for each job.
For that, you have to edit the details of the line concerned by clicking on the pencil on the right and define the discount (in %). As you change the percentage, the line total will update automatically.