In PTC there are links that earns credits instead of cash. What are credits for?

@iskayz (5420)
Philippines
September 7, 2008 5:43am CST
Hi everyone! Can anyone help me? In some of my PTCs there are links that earns credits instead of cash. But I don't know how to use those credits. What are they for? How and where can I use it cause some aren't convertible to cash.
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