We use the phrase source program, abbreviated source, to describe the program input to one run of the assembler. The program may be in one or more files; how the source is partitioned into files doesn't change the meaning of the source.
The source program is a concatenation of the text in all the files, in the order specified.
Each time you run the assembler it assembles exactly one source program. The source program is made up of one or more files. (The standard input is also a file.)
You give the assembler a command line that has zero or more input file names. The input files are read (from left file name to right). A command line argument (in any position) that has no special meaning is taken to be an input file name.
If you give the assembler no file names it attempts to read one input file from the the assembler standard input, which is normally your terminal. You may have to type Ctrl-D to tell the assembler there is no more program to assemble.
Use -- if you need to explicitly name the standard input file in your command line.
If the source is empty, the assembler produces a small, empty object file.
There are two ways of locating a line in the input file (or files) and either may be used in reporting error messages. One way refers to a line number in a physical file; the other refers to a line number in a "logical" file. See Section 2.5.
Physical files are those files named in the command line given to the assembler.
Logical files are simply names declared explicitly by assembler directives; they bear no relation to physical files. Logical file names help error messages reflect the original source file, when the assembler source is itself synthesized from other files. See .app-file.