I have to confess I've never used this. Presuming you have worked out how to read standard variables passed to your ASM function, it would just mean POPing the 16bit segment before (or after?) the 16bit offset.
SEG---Indicates the parameter is passed as a segmented address (far pointer).
Your probably the only QBASIC programmer actively using the SEG prefix, great stuff!
We'd be able to help you more if you showed us you ASM code as well as your QBASIC code.