The Forum has nested icons on every thread, and these icons give you a hint about what is happening in the thread. However, some newbies and perhaps other members don't look at them, and they don't understand what they represent even though they have hints under the navigation of all the boards that we have in the Forum. The available thread icons include the following:
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 | Topics you have posted in: This icon often appears in threads in which you have engaged or replied, they will appear in front of any thread in which you have submitted one or more comments. It is important to always check for this icon in order to avoid spamming and the duplication of various replies to the OP in a thread. | |
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 | Normal topic: This icon tells you that you have not responded to the thread before and the thread is still on its first page. When you want to reply or interact in any new threads on any board, these are the type of kind of threads to engage with for a start | |
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 | Hot topic (more than 21 replies): This icon usually displayed in threads that are more than one page. When discussions are more than one page, it usually changes to this icon. | |
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 | Very Hot Topic (more than 100 replies): This icon is usually displayed in threads that have more than one page. When discussions are more 4 one page, it usually changes to this icon. | |
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 | Locked Topics: Threads can be closed for a variety of reasons, depending on the OP, such as to prevent spamming or to prevent other users from contributing to the thread. When you see this padlock icon on a thread, it means it is locked and you cannot reply to it. | |
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 | Sticky topics: This icon is normally placed at the top of the boards, they are used to include vital information and, in order to prevent navigating to the next page in the board, the moderators used to attach them to the top of each board for users that need them for quick information. | |
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 | Poll Icon: This is primarily for voting, the posts are normally to seek user opinions to vote on the options that are provided by the OP, whenever you see this type of thread, just know that you are required to vote and you may or may not reply. |
There are other Icons for threads but these are the available icons for threads with the basic information I have seen in bitcointalk forum, I hope this helps people that are new to the forum and those who are just understanding them for the first time. Cheers.