This XFCN sublaunches the program with optional attached document. Both HyperCard and the specified program will then be running, memory permitting (at least 2 Meg!). If the arguments have no colons in them (not a pathname), then the “Look for applications in:” etc. cards in the Home stack are consulted.
SubLaunch return 0 for success, negative numbers for Operating System errors, and small positive numbers for parameter errors, or lack of memory.
One should be familiar with Apple's Tech Notes #169 about bugs with HyperCard 1.0.1 in the Multifinder environment.