This advanced option defines the size of the memory cache used to store rebar set bars. When you increase the size, more rebar set bars can be kept in memory. This means that the rebar set bars are not regenerated so frequently, which will improve the performance. For optimum performance, the size should be greater or equal to the number of rebar set bars in the model.
This advanced option is user specific and the setting is saved in options.bin under user folder. Restart Tekla Structures to activate the new value.