What is the difference between TT and OD currency exchange rates?

I was looking at Singapore dollar exchange rates provided by a bank:

Source: Foreign Currencies – Foreign Exchange | POSB

What is the difference between TT and OD?
The webpage itself says:

• Telegraphic Transfer ("TT") rates and On Demand ("OD") are rates available involving foreign exchange.

• The TT rate is applicable to funds that has already been cleared with the Bank while the OD rate is applied otherwise.

• The buying rate is used when foreign currency is sold to the Bank and the selling rate is used when foreign currency is bought from the Bank.

I don’t understand the "explanation" above. I also tried to read other websites, but all of them go on and on about remittances, demand drafts, foreign checks, invoices, bills of exchange, letters of credit, … which are documents and instruments that I don’t really understand.

• What is TT and what is OD?
• Suppose I have Euros, and walked into the bank to buy Singapore dollars, which rate will I get? $1.5809 per Euro or $1.5731 per Euro?
• Notice how the TT rates are generally better than the OD rates. Why is this so? And why is the US dollar alone in having a TT rate that is worse than the OD rate?
• Since TT rates are usually better, how do I get TT rates?