Just as a reminder, if you're adding a magazine issue, do remember to fill in all ItemCats for the item; they're all important, as blanks or incorrect values for ItemCats can be seen both on monthly stat reports, and in the catalog's filters.
ItemCat1 is for the broad type of material. Monthly stats rely on ICat1 to tell you generally what's circulating at your library.
ItemCat2 is for the age group for the material. Monthly stats also rely on ICat2 as a secondary breakout, so that you could tell how many Adult books were checked out as opposed to Juvy books.
ItemCat3 is for if the material is Fiction or Nonfiction (or other). The catalog relies on ICat3 to allow for patrons to narrow their searches based on if they want books on (for example) how to knit, or cozy mysteries with knitting.
ItemCat4 is for state stats, and reflects if the item is something the state considers a circ, or if this is an item the library wants to keep track of through a checkout; most items have an ICat4 of CIRC, and a very few have an ICat4 of INHOUSE.
Generally for magazines, ItemCat1 should be MAGAZINE. ItemCat2 should ADULT, YA-TEEN, or JUVENILE. ItemCat3 should likely be NONFICTION (though there may be a magazine that is entirely short story subscriptions, and so could be FICTION). ItemCat4 should be (and defaults to) CIRC.