AT&T 6300 Programmer's Manual page 401

Remarks
Example
PLAY
Statement
The "n" parameter may be constant or
variable, where a variable is written as:
"+variable;." The semicolon is necessary when
a variable is used in this way, or when the X
command is used, but it is not allowed after
MF, MB, MN, ML, or MS. In all other cases, a
semicolon is optional between commands.
When the X command is used, VARPTR$
(variable), may be substituted for "variable;" as
in the example below.
100 PLAY u02 L4 C P1 C P2 C P4'
200 PLAY uXBS;XCS;XDS;'
300 PLAY uXMS;'
or
300 PLAY uX"
+
VARPTRS(MS)
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