yes when i say shared hosting i mean exactly what you are referring. We hosting providers offer multiple domains to be hosted on single shared hosting account, some offer unlimited addon domains to that single shared hosting account. see the image for the first plan of namecheap
http://prntscr.com/n6dtkp
every shared hosting account has resource limits which are tuned by hosting provider, lets say i have 3 plans in first very basic plan i offer 1 gb memory 1 cpu core,
and in second plan i offer 2 gb and 1.5 cores and so on, but i also offer extra domains can be hosted in same basic shared hosting plan,
now you bought the basic plan with primary domain xyz.com resources remains the same 1 gb 1 core. but then you added extra domains in same shared hosting account as addon domains, remember your package is same 1gb 1core basic shared hosting, as i said in my previous reply hosting providers are using cagefs so the account will be strict to use that single core and 1 gb ram, you will end abusing resources or slow website at the end.
what i simply mean is just buy multiple shared hosting accounts for each of your domains.
PS: my site is theonionhost.com