I studied the hell out of this today so I can take a stab at this
SVC can be caused by a pancoast tumor OR a mediastinal mass
Pancoast tumor (mass at the apex of the lung) is commonly referred to as "superior sulcus tumor" because a mass at the lung apex will have a mass-compression effect in between the subclavian vessels.
If you have a right sided apical mass that is large enough it may compress the superior aspects of the SVC (i.e., not in the proximity of where the SVC meets the RA in the mediastinum); in that case, you have an obstruction to venous drainage of the head, neck, and both arms because SVC is the outlet for all that.
If you have a right or left sided apical mass (pancoast tumor/superior sulcus tumor) of a more modest size, then it is more likely going to produce an ipsilateral horner syndrome or compression/invasion of the brachial plexus, causing frank deficits in the distribution of the brachial plexus.
If you have a mediastinal mass: the SVC meets the RA within the mediastinum; the heart is a mediastinal structure. If you have a mass in the mediastinum that is compressing the SVC, then you will haven an obstruction to venous drainage of the head, neck, and both arms because SVC, again, is the common venous drainage outlet for those structures.
Finally, if you have a brachiocephalic vein compression (keep in mind that the SVC is formed by the union of the right and left brachiocephalic) you basically get a "1/2" of a SVC syndrome - if the left BC vein is compressed, you'll have the symptoms only on the left (left sided plethora, flushing, etc); likewise, if the right is compressed, symptoms look like a right-sided SVC syndrome.
TL;DR - Mediastinal Masses = can cause SVC syndrome. Apical masses more likely to cause ipsilateral Horner syndrome & brachial plexus defects, but if on the right side and large enough, may cause SVC syndrome.