I am using Proteus 8.7 and I am so confused about how to make a power plane (copper pour) exposed . I mean I want that area free from green mask so that It can make its use as thermal dissipation area. Which layer should I use to make this opening ? The layers I can use for the top side is "top paste" and "top resist" for this purpose . But I don't know which one would help me to achieve what I want .
The paste (more properly solder paste) layer defines where solder paste is to be printed on the board during assembly. It's used to make the solder paste stencil.
The resist (more properly solder resist) layer is what you want. "Solder resist" is synonymous with "solder mask". I'm not familiar with Proteus, but usually solder resist layer is negative, meaning that if there's something drawn on that layer, then that's where solder mask will be missing in the completed design.