A standard UML2 «subsystem» is a "logical" grouping of functional requirements represented as Use Cases. It does NOT (usually) to the "physical" software Classes/Component of a software system that collaborate to provide users with functional services.
A SysML «subsystem» is a non-normative extension of block; it is a "physical" grouping of blocks within a SysML«system».
TIP: Although not standard SysML, you MAY sensibly combine the standard UML subsystem and the non-normative SysML "physical" subsystem concepts in one M D SysML model.