Below is a good case study on the subject from the Revenue Dept ruling, Gor Khor 0811/758, dated 29 January 2001 

Re: Permanent establishment in holding a seminar in Thailand 

The company was set up under the law of Philippines. It had no branch office or agent in Thailand and did not register itself as a VAT trader in Thailand. The company arranged a seminar - The Seventh Annual Fuels and Lubes Asia Conference - in Thailand and sent speakers and staff to Thailand for a period of 3-4 days. The company received income in the nature of registration fees from local and foreign attendees. 

The Petroleum Authority of Thailand sought advice how it should withhold tax on the registration fees. 

The Revenue Department made the following decision: 

1 Corporate Income Tax - Since the registration fees for attending the seminar organised by the company in Thailand was treated as business profit, the company was not required to pay Thai corporate income tax in the absence of the permanent establishment, pursuant to Articles 5 and 7(1) of the tax treaty between Thailand and Philippines, and Section 3 of the Royal Decree No. 18. As a result, PTT was not required to withhold tax on payments to the company. 

2 Value Added Tax - Since the company was an offshore trader and conducted business in Thailand temporarily without VAT registration, pursuant to Section 85/3 of the Revenue Code, PTT was required to pay VAT by remitting the amount to the Revenue Department via Form Phor Por 36 within seven days of the following month after payment, pursuant to Section 83/6(1) of the Revenue Code.