Benefits for tax directive

I am a commissioner earner and received a tax directive for 18% from SARS for the 2015 tax year. What benefits are there to having a tax directive? Will I have to pay back on my tax return or will SARS calculate my tax at 18% according the tax directive?

You may have to pay in to SARS as the directive only states that tax be withheld at 18%. However SARS will take your overall income less allowable expenses and then determine the tax which may or may not be more than 18%.