Significance of numbers in server errors?

I'm wondering if there's a description anywhere of the significance of number
reported in errors; for example I've recently run into this error:

ERROR:  could not read block 132 of relation 1663/16430/1249: read only 0 of
8192 bytes

From some documentation I've read

(http://etutorials.org/SQL/Postgresql/Part+I+General+PostgreSQL+Use/Chapter+4.+Performance/How+PostgreSQL+Organizes+Data/)
 I know the second is the database's directory, the last is the pg_attribute
table (in this example), but I haven't figured out what the first is.

Additionally, is that format usually consistent across errors? For example,
would the number in this error relate to the same values:

ERROR:  could not open relation 1663/16430/16868: No such file or directory