Planets operate differently according to their position to the cusp of the houses.
The closer the planet is to the degree of on the cusp (either from one side or the other - from 12th or 1st) - the more powerful and precise is its involvement with the affairs of that house.
Eg. - Mars in 12th house close to the Asc and Mars in 1st close to the cusp.
Astrologers often say - planet that makes an exact trine to the Asc is very strong.
Greater influence and stronger placement for a planet is in a house is close to the cusp.
When a planet is in the middle of the house - its influence diminishes rather than increases.
So, regarding Mars close to the cusp of the beginning of the 1st house will have an affinity with the 1st ,3rd house and 8th (trine - Mars rules Aries and Scorpio which are 1st house and 8th house)
Another interesting information about subdivision of the houses in the chart, they divided into 4 additional trinity:
Houses 1, 5 and 9 Aries, Leo and Sagittarius Spiritual action trinity Houses 2, 6 and 10 Taurus, Virgo and Capricorn Wealth Houses 3, 7 and 11 Gemini, Libra and Aquarius Relationships Houses 4, 8 and 12 Cancer, Scorpio and Pisces Soul.
Fire planets houses (Spiritual action trinity) – planets in those houses effect the attitude towards life itself and entire sense of confidence in ourselves. His/her enthusiasm, ideas, dreams for the future.
Earth planet houses (Wealth) – effect our attitude towards career, ambitions, how to produce effective results. Those tend to really settle to in life, it's like they are searching for ways to be most productive and easier satisficed trough their practical needs.
Air planet houses (Relationships) – strongly effect our interest and associations, mood of verbal expression and our social life. They tend to live in mind and in relationships.
Water planet houses (Soul) – effect our emotional predisposition and how we are copying with them, fulfilling our privet needs, and confronting our obsessive feelings as well. Someone with emphasis on water houses, maybe have a great deal to live in his/her feelings, deeper earning.