Both of them are different and not similar.Reference lines in Tableau are used to show visual comparisons to benchmark certain figure, constant or values. These can be added at a constant or computed value on the axis. For example, we are showing sales of Firm1, Firm2 and Firm3; the reference line can be set as an “average sales figure” which could act as a benchmark. Trend lines on the other hand can be used to represent pattern of the trends in the data. For the same above example of Sales of a Firm1, we can use different trend lines to represent if it is following a particular trend. These can be linear trend, logarithmic trend, exponential trend, polynomial trend, power trend etc.