TA (type approval) refers to any variation of where something has a mandatory level of required tested certified conformity in respect of how it’s designed for use, and EM emissions type and level associated with it.
For simply being legal to import into a nation and it’s overseas territories or into an economic trade region (such as the EU) often requires such qualifying goods to at least be capable of meeting the basic criteria of conformity for any radio service(s) it’s designed (specifically or generally inclusive of) for use with.
For sale/supply for a user service irrespective of category of user service, it would need for the manufacturer or importer (typical the manufacturer, unless it’s a repurposed use item, where importer takes liability for TA declaration and claimed conformity) to seek certification for every single TA required marketed service use it’s sold/supplied for in the territory/economic regions.
So that, in a nutshell, defines how TA affects the legally obtainable supply of such goods in your region or economic area. So, depending on region, it may be legal to pre-import pending specific or grouped bulk services certification - but at best, this only really is valid on equipment where there are supply exemptions (such as with RF dev kit systems, reference system examples, and other aspects where permitted test/eval and experimentation/development is permitted by developers and subcontractor manufacturers/OEM) and in most cases ‘certification pending‘ compliance in principle import clearance is only truly legit once the manufacturer or importer has sought permission to pre-source and stock pending getting regional/EA certification. So unless it qualifies for development exemption by design (ham radio market kit falls into this category), it’s not even legal to commercially import in most regions/EA’s pre to certification.
That’s why many dodgy devices get imported with a large network of individuals who effectively personally import limited quantities as 'personal imports’, under one financial supported model or another. In the UK, pre to the effectively defunct MPT1320 certification, many illegal multimode and mono mode FCC spec CB rigs got brought in to sell under the guise of personal imports.
The status, for sale/supply in the region/EA where pre-certification imported, of goods which are sold but never certified (outside of limited exemptions) is pretty clear as is the liability for vendors of and end users - not a good place to be.
Where pre-certified (samples have under gone applicable regional/EA certification) or certified whilst pending sale/supply once imported in principle reach valid TA stage, how and where it’s valid to sell supply actually depends on it’s specific TA status and support subsequent usage of.
In simple generic terms these statuses fall into one of these groups -
Certified (meaning currently so) - where, subject to any further limitations governing who can sell/supply, usage is permitted under licensed, license free or license exempted status whilst the certification remains valid.
Certification suspended typically refers to where equipment has it’s certification suspended (and such it’s compliancy status) pending a reevaluation process or where operationally new user permits (licenses, formal or implied) are suspended. Where it’s a license issue based suspension, existing permitted use equipment and licensed use usually remains legal until further ratification has taken place - such as allocations being subject to pending alteration or when services are combined and revamped.
Certification revoked literally means it’s neither certified any longer, meaning sales/supply of functioning equipment and subsequent use of obtained or owned items become illegal where permitted sales/supply/usage depends on a valid current certification of the goods.
Certification requirements dropped literally means there is no longer a requirement to ensure you sell/supply goods that are current to applicable certification and end users retain licensed/permitted usage as previously (the terms of the license/permit remain applicable) was the case. These are, in the UK, very rare cases - it’s more likely (as happened to CB radio) where operator license holding was no longer required but as a result, all previous certification requirements tied to the defunct license remain in effect.
So, without really touching on regional/EA specific formal designated examples, that’s the significance of TA as far as the Sale/supply/usage of em-generating goods are concerned, where it covers both em-emmision by design (radio/wireless EM) or has a notable potential for creating subsequent EM emissions beyond it’s actual design purpose.