Membership Start Date vs Subscription Start Date
Membership start date shows the day in which your member has participated in your organization. It's totally up to you to enter this information. It's only for your records, so you can leave it blank.
However, subscription start date is the date on which your member is included in a plan. It's necessary for the automated payment process and the reminder emails.
The significance of having a subscription start and end date
Subscription start date is necessary to define the subscription period of your member. For example, if you create your plan as "yearly", the debt for the next year will be assigned to your member on the same day. If you don't enter a specific date while your subscription import, subscription start date will be the date on which you include your member in a plan.
If you enter a subscription end date, the system will not assign the debt for the new session. So, it is better to leave it blank for the automated process.
Membership/Profile Status vs Subscription Status
Membership profile status can be "Active, Deceased, Resigned, Frozen". It's independent of the Subscription process. A member can still be an active member even though he/ she doesn't pay the subscription fee and his/her subscription staut turns into past due.
Subscription status can be active, past due, frozen or canceled. This status is updated according to the payments of your members in the automated payment process.
Auto update of membership or subscription status when an overdue payment is made/recorded
When an overdue payment is recovered in the automated process (the system will try to withdraw the failed payment 5 times), the subscription status will be updated automatically as active. But the membership status is totally in your control. It is independent of this automated payment process.