What is the difference between a LOAD COMBINATION and a REPEAT LOAD?
The difference lies in the way STAAD goes about calculating the results - joint displacements, member forces and support reactions. For a load combination case, STAAD simply ALGEBRAICALLY COMBINES THE RESULTS of the component cases after factoring them. In other words, for example, in order to obtain the results of load 10, it has no need to know what exactly constitutes load cases 3, 4 and 5. It just needs to know what the results of those cases are. Thus, the structure is NOT actually analysed for a combination load case. With a REPEAT LOAD case however, the procedure followed is that which occurs for any other primary load case. A load vector {P} is first created, and later, that load vector gets multiplied by the inverted stiffness matrix to compute the displacements. In other words the structure is analyzed for the combination of loads rather than just combining the results.
The following wiki describes how a LOAD COMBINATION can be easily converted to a REPEAT LOAD
https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/30456/how-to-convert-a-load-combination-to-a-repeat-load