The format rule includes all grammatical aspects of paragraph structure. We review below the paragraph structure of Lv15.

And the Lord spoke to Moses and to Aaron, saying, Speak to the people of Israel, and say to them, When any man has a discharge out of his flesh, because of his discharge he is unclean. And this shall be his uncleanness in his discharge; whether his flesh runs with his discharge, or his flesh is stopped from his discharge, it is his uncleanness.
1. Every bed, on which he, who has the discharge, lies, is unclean; and everything, on which he sits, shall be unclean.
   • And whoever touches his bed shall wash his clothes, and bathe himself in water, and be unclean until the evening.
   • And he who sits on any thing on which he, who has the discharge, sat, shall wash his clothes, and bathe himself in water, and be unclean until the evening.
2. And he who touches the flesh of him who has the discharge shall wash his clothes, and bathe himself in water, and be unclean until the evening.
3. And if he who has the discharge spits upon him who is clean; then he shall wash his clothes, and bathe himself in water, and be unclean until the evening.
4. And whatever saddle he, who has the discharge, rides upon, shall be unclean.
   • And whoever touches any thing that was under him shall be unclean until the evening; and he who carries any of those things shall wash his clothes, and bathe himself in water, and be unclean until the evening.

The Rashi comment is simple, straightforward and based directly on the bulleted structure: The chapter enumerates 4 methods of communication with ritual uncleanness for a person with excessive discharge: his bed, his touch, his spit, his riding. Notice how the ride and bed sections have sub-bullets while the touch and spit section to not have sub-bullets. We see all 4 methods of communication make the communicated person ritually unclean. However we additionally see that lieing / riding have the power to make a person touching the bed unclean. In other words the bed becomes a primary transfer (Father) of ritual uncleanness while the the touching only creates recipiency of ritual uncleanness but not the ability to transfer.

Summary: Whether you get spit at or touched or lied on by the person with discharge you become ritually unclean. However a person spitted on or touched cannot further transfer his ritual uncleanness to a 3rd person. By contrast the bed on which the person with discharge sleeps does have the capacity to transfer ritual uncleanness to a 3rd party. This distinction between touching and beds is conveyed by the sub-bullet structure.

Although we have been somewhat technical in this Rashi it is noteworthy that the Rashi comment is solely derived through analysis of structure. The Rashi comment is not derived through word meaning or grammar.