Radio Shack TRS-80 PC-3 Owner's Manual page 121

The third form of CHAI N searches the tape for the program whose name is indicated by "filename", loads the program, and begins
execution with the lowest line number.
The fourth form of CHAIN will searches the tape for the program whose name is indicated by "filename", loads the program,
and begins execution with the line number indicated by the expression.
Examples
10
CHAIN
20
CHAIN "PRO·2", 480
Loads the first program from the tape and begins execution with the lowest line number.
Searches the tape for a program named PRO-2, loads it, and begins execution with line number 480.
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