Need to read it carefully. For example, a particular policy might be must-issue and still exclude pre-existing conditions, so that the insurer must sell you a policy but can deny all claims for pre-existing conditions. Generally this is for a limited time, typical 12 - 24 months. A pre-existing condition ban means that insurers cannot exclude coverage for a condition if it existed prior to the insurance application being submitted. You get a policy, it immediately covers your pre-existing condition costs. And of course, guaranteed availability does not mean you are going to get a policy, merely that insurers can't sell your neighbor a policy and refuse to sell you the same policy if you qualify. It's meant to stop discrimination, not guarantee you a policy.
Community rating means your premiums must be the same as everyone else's with the same risk factors, which may be everyone with a pulse or may consider many factors. It may require your premiums to be the same as everyone in your community, but not necessarily. A modified community rating still allows for different rates to be charged. For instance, Obamacare allows smokers to be charged more. If you are a smoker with emphysema, your premiums must be the same as those of a smoker without emphysema, but not necessarily the same as those of a non-smoker.
Read Sactoking's response carefully and you'll see the differences between terms.