Italian tablature was the most common style of tablature used by the Spanish vihuelists and four-course guitarists during the SCA period. Mudarra, Fuenllana, Milan, etc. all used Italian tablature in their compositions. The tablature lines are arranged such that:
Unlike French tablature, Italian tablature uses numbers to represent the frets of the instrument.
For example, the number “1” on the lowest line of the tablature indicates that the performer should play the first fret of the first string/course (the note “F” as played on a modern guitar).
The description above applies to most of the Italian tablature sources. There are a few exceptions. Luis de Milan's vihuela book, "El Maestro" from 1536, uses Italian tablature as described above, except that the tablature lines are opposite of the orientation described above. Like French tablature, Milan's tablature is organized such that the top line of the printed page represents the highest pitched course of the instrument, and the bottom line represents the lowest pitched course - the tablature of Milan is very similar modern guitar tablature, and is a sort of hybrid between typical Italian tablature and French tablature. Likewise, the 4-course guitar pieces included at the end of Barberiis' 1549 book also use this "hybrid" style of Italian tablature.
Example of Italian tablature, from a 4-course guitar pavan by Alonso Mudarra (1546)
Keep in mind that these "note values" do not necessarily indicate note duration. Rather, the tablature simply shows the performer the timing between one note and the next. It is up to the performer to interpret how long to hold a note. A general rule of thumb is to hold the note as long as possible, until the tablature later requires that another note (on the same string) takes its place. There are frequent instances, in my opinion, where this rule of thumb should be ignored. Sometimes, holding a note as long as possible results in dissonance where dissonance is clearly not intended by the composer. This is a drawback to the tablature system, but it does make interpreting the period compositions more interesting (at least, I think it does!). Since the tablature system does not depict exactly how long a note should be held, it is up to the performer to interpret the music. A best practice is to hold an individual note as long as possible, unless the music gives a good reason to not do so.
An example of Italian Tablature, from Miguel de Fuenllana (1554). Compare the Italian Tablature (uppermost line) to the transcribed staff notation and modern guitar tablature.