The members of this object are the arguments passed to your wrapper function. If you are outside a wrapper function, this object has no members. You cannot add or delete members from this object, you can only access and update the actual parameters passed to your function.
Since ScriptEase supports named arguments, you can specify a normal member name. However, most wrapper functions have no names for the arguments, and names are only present if the caller provided names for the arguments. Normally, you just use SE_NUM(x) to access argument number x. For instance, SE_NUM(0) is the 0th argument (i.e. the first argument passed to your function) and so forth. For the arguments object, SE_INDEX accesses the members just like SE_NUM does, they are synonymous.
You cannot use SE_VALUE with SE_ARGS.