daf bit: Chullin 25

The mishna describes a series of "either-or" cases. From yesterday's daf we learn: a disability that disqualifies priests does not disqualify levites and one that disqualified levites does not disqualify priests. Also, that which can be rendered unclean in earthenware vessels cannot be so rendered in any other vessels and vice-versa. Today we learn: that which cannot be rendered unclean in wood can be in metal and vice-versa; when bitter almonds are subject to tithe sweet ones are not and vice-versa; brothers liable to the "agio" (a certain tax) are not liable to the cattle tithe and vice-versa. (24-25)

I have not yet seen a general principle or explanation.