Top 30 is the first 30 stocks in the result section sorted by the criterion you've chosen. For example, if you sort the result by Jitta Ranking, Top 30 is the first 30 stocks in the result section as ranked by Jitta Ranking. If you sort the result by Jitta Score, then Top 30 is the first 30 stocks in the result section as ranked by Jitta Score. The annual returns produced by these two cases would differ.
If you want to invest following your Jitta Playlist, you must invest in the first 30 stocks listed on your criteria. How often you should rebalance your portfolio depends on your fee management and risks taking. If your choice of stocks are mostly rated 5 then it is deemed as very risk and may require more rebalance intervals (Every 3 Months or 6 Months). However if Jitta Score is above 7 you can hold for a minimum of 1 year and rebalance after if you feel stock performance isn't up to its original standard.