sādhaka-rūpeṇa yathāvasthita-dehena |
siddha-rūpeṇa antaś-cintitābhīṣṭa-tat-sevopayogi-dehena |
tasya vrajasthasya nijābhīṣṭasya śrī-kṛṣṇa-preṣṭhasya yo bhāvo rati-viśeṣas tat-lipsunā |
vraja-lokās tv atra kṛṣṇa-preṣṭha-janās tad-anugatāś ca tad-anusārataḥ

Sādhaka-rūpa refers to the physical body of the practitioner. Siddha-rūpa refers to the body which is suitable for one’s desired service, and which has been developed by internal meditation. One serves with a desire for the particular bhāva or rati of an associate of Kṛṣṇa situated in Vraja (tad-bhāva-lipsunā). One should follow in the footsteps of the dear associates of Kṛṣṇa in Vraja (vraja-lokā) and others loyal to them.

(Srila Jiva Goswami, tika to BRS 1.2.295)

An example:

So, what we are doing is like making a mental sculpture in the anugatya of Sri Gurudeva and Krishna spiritualizes it through the power of bhakti. When our mental sculpture is reaching perfection (rati, asakti, bhava), it is not a mental sculpture anymore but a spiritual reality by the power of bhakti.

And to gain entrance into the spiritual world, Krishna awards us with this internally developed siddha deha according to our love.

tvaṁ bhakti-yoga-paribhāvita-hṛt-saroja
āsse śrutekṣita-patho nanu nātha puṁsām
yad-yad-dhiyā ta urugāya vibhāvayanti
tat-tad-vapuḥ praṇayase sad-anugrahāya
(Srimad Bhagavatam 3.9.11)

“O Lord! You, who are approached by being heard about, seen and directly served, enter and remain in the lotus of your devotee’s hearts infused with bhakti-yoga.
Much praised Lord! By your mercy, you bestow to them spiritual bodies appropriate to the mood they cultivate during sādhana.”

Srila Viśvanātha Cakravartī Ṭhākura explains – te sādhaka-bhaktāḥ sva sva bhāvānurūpaṁ yad yad dhiyā bhāvayanti tat tad eva vapus teṣāṁ siddha dehaṁ praṇayase prakarṣeṇa tān prāpayasi aho te sva bhakta pāravaśyam iti bhāvaḥ –
“Or your devotees doing sādhana concentrate on their spiritual form according to their mood of love using their minds, and you have them attain (praṇayase) their spiritual bodies. You become dependent on your devotee.”