Regarding the applicable scope and market conditions of Dow Theory, we can make the following analysis:

1. Major Trends

Dow Theory is most applicable in grasping the "major trends" or simply "trends" in the stock market. The stock market can be divided into major trends, intermediate trends, and daily short-term fluctuations. These concepts of major trends, intermediate trends and short-term movements were originally proposed by Dow Theory. The theory is particularly useful in identifying major market trends. By following Dow Theory's investment approaches during major market trends, investors can easily profit or at least minimize investment risks.

2. Long-term Investments

Dow Theory excels at identifying major market trends, namely the bull and bear markets proposed by Dow. Even small bull/bear market cycles typically last several years, while larger ones may extend over ten to twenty years. Dow Theory provides exceptionally detailed and insightful analysis of these large-scale bull/bear markets, making it most instructive for long-term stock investments.

3. Intermediate Trends Also Useful

Dow Theory is also valuable for analyzing intermediate market trends. Beyond major trends, the stock market also experiences intermediate trends within larger trends. Dow Theory describes the characteristics of these intermediate trends and suggests appropriate investment strategies. For example, investors should buy during rebounds in the second phase of bear markets, and exit or short near the end of bull market's third phase. Though these are intermediate-term strategies, Dow Theory remains applicable.

4. Short-term Trading Still Has Reference Value

Dow Theory primarily focuses on major trends and their intermediate movements, providing limited guidance for daily fluctuations. Therefore, its application is more limited for short-term or day trading. Nevertheless, Dow Theory still offers reference value for daily trading.

For instance, a futures trader employing short-term strategies - buying today and selling tomorrow or even closing positions intraday - still needs to reference whether the current market is bullish or bearish. In bull markets, long positions can be more aggressive, while bear markets warrant more confident short selling. Thus, even ultra-short-term day trading requires understanding Dow Theory's market phase analysis.

5. Also Applicable to Derivatives Analysis

As the stock market evolved, various derivative instruments emerged whose prices fluctuate with underlying stocks. Therefore, understanding Dow Theory's market analysis is equally essential when investing in any stock-related derivatives - including index futures, warrants, put warrants, stock options, index options, etc. Correctly analyzing current market conditions is absolutely critical for all stock market participants.