Hi
The solid226 are coupled (multi) physics elements (so e.g., can be thermal+structural, thermal-electric, piezoelectric, etc.).
Solid186 are single physics, pure structural elements (so used only for structural analysis).
The element shape can be as you say HEX20 for both of these elements – in addition we can have wedge, prism, tet and pyramid shapes for both these element types (solid186 and 226). So the answer to your questions is both.
So we use solid186 (default in static structural) if we are doing say a pure structural analysis. So Ansys will choose the correct element (solid226 or solid186) depending on the type of analysis used. So again if a static analysis is used, then solid186/185 are used, and if thermal-electric system/analysis is used then soild226 elements are used by Ansys.
See the element reference manual inside ansys help for more info.
All the best
Erik
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