I usually see this on tables or forms where some are using dropdown and some are using "unfold more" icon. Is there any difference or each icon has its own meaning?
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To my understanding, they all are sorting icons that represent different state of the sorting. These images were taken from font-awesome.
In the case of a drop-down, it doesn't make any difference which icon you are going to use, as long as it delivers the message that it is a drop-down.
Both of what you have shown work since they are quite commonly used. Try to keep it consistent with the whole system.
I'm not sure if I get the second icon right, but it looks to me like it's two icons; expand and collapse view. In that case, the one on the left is the same as the bottom one on the right.
If that's not the case, the two icons are very different. The one on the left is to indicate a dropdown menu and the one on the right can increase or decrease a value. For example in a spinner or slider.
In my experience the caret in a user interface simply means "more". It can be used as a toggle to expand/collapse a section, as a dropdown icon, or in a content slider to indicate more content to the right (or left). The direction of the "more" is usually indicated by the direction the caret is pointing.
See this answer: https://ux.stackexchange.com/a/21531/9932
And this answer: https://ux.stackexchange.com/a/96650/9932
In my opinion, those icons are different.
I have never seen a sort icon that uses chevrons, I have only seen it using arrow triangles.
The expand chevron indicates hidden content that typically appears below something, for example a heading in a box which is expandable and collapsable, for example an accordion.
The unfold icon to me indicates that you can unfold content inbetween content. For example when you search for a public transport trip and you'll first see only start and end destination. I would use this icon to unfold the stops inbetween start and end destination.
It completely depends on the context of usage.