What is a B2B ecosystem?

Short answer

A B2B ecosystem is a group of business applications that belong to one platform and share the same account, company record and customer record, so information entered once is available in all of them.
It differs from a software suite because the modules are activated one by one and billed as a single subscription: a company starts with one and adds the rest when it needs them.
It differs from an app marketplace because the integration is not the customer's job: the modules are built to exchange data, not wired together with external connectors.
For a small company the measurable gain is the end of re-typing data between applications and of paying to keep integrations alive.

Separate applications: each has its own account, subscription and API, and data moves by hand or through paid connectors.
Integrated suite: one product with inseparable modules — you buy the whole thing even if you use a third of it.
App marketplace: a large catalogue, but the integration and its upkeep stay with the customer.
Ecosystem: modules that are commercially independent and technically connected — one account, shared data, activation in stages.

An ecosystem does not change a company's tax duties: in United States the IRS filing rules apply exactly as before.
Where electronic invoicing is used, the local channel decides the format — in United States that means voluntary e-invoicing exchange over the DBNAlliance network and state sales-tax filing.
The company details printed on a document must match the record held by the Secretary of State.
Data-protection rules ask where records are stored and who can read them; an ecosystem answers that once, not six times.

Example: a customer added in the CRM is already filled in when the invoice is issued and when the contract is signed — the tax number and the address are not typed three times.
Example: a services company starts with invoicing only, adds the CRM two months later and time tracking after its first hire — with no data migration between products.

Treating "integrated" and "ecosystem" as the same thing: two apps joined by a connector are still two products, two subscriptions and two points of failure.
Choosing by the number of modules instead of by the modules you will actually use in the first year.
Ignoring the way out: ask for the export format of your own data before you sign up.

Modules are activated individually — you pay for what you use, not for a bundle.
Company and customer records are entered once and valid in every module.
One login for the whole business surface, with permissions per role.
Local e-invoicing formats and channels are included on every market that is open.
Data export in standard formats, so you are not locked to one supplier.